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2019-05-28rcu: Make __call_srcu staticJiang Biao
Because __call_srcu() is not used outside kernel/rcu/srcutree.c, this commit makes it static. Signed-off-by: Jiang Biao <benbjiang@tencent.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2019-05-28srcu: Allocate per-CPU data for DEFINE_SRCU() in modulesPaul E. McKenney
Adding DEFINE_SRCU() or DEFINE_STATIC_SRCU() to a loadable module requires that the size of the reserved region be increased, which is not something we want to be doing all that often. One approach would be to require that loadable modules define an srcu_struct and invoke init_srcu_struct() from their module_init function and cleanup_srcu_struct() from their module_exit function. However, this is more than a bit user unfriendly. This commit therefore creates an ___srcu_struct_ptrs linker section, and pointers to srcu_struct structures created by DEFINE_SRCU() and DEFINE_STATIC_SRCU() within a module are placed into that module's ___srcu_struct_ptrs section. The required init_srcu_struct() and cleanup_srcu_struct() functions are then automatically invoked as needed when that module is loaded and unloaded, thus allowing modules to continue to use DEFINE_SRCU() and DEFINE_STATIC_SRCU() while avoiding the need to increase the size of the reserved region. Many of the algorithms and some of the code was cheerfully cherry-picked from other code making use of linker sections, perhaps most notably from tracepoints. All bugs are nevertheless the sole property of the author. Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> [ paulmck: Use __section() and use "default" in srcu_module_notify()'s "switch" statement as suggested by Joel Fernandes. ] Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
2019-03-26srcu: Remove cleanup_srcu_struct_quiesced()Paul E. McKenney
The cleanup_srcu_struct_quiesced() function was added because NVME used WQ_MEM_RECLAIM workqueues and SRCU did not, which meant that NVME workqueues waiting on SRCU workqueues could result in deadlocks during low-memory conditions. However, SRCU now also has WQ_MEM_RECLAIM workqueues, so there is no longer a potential for deadlock. Furthermore, it turns out to be extremely hard to use cleanup_srcu_struct_quiesced() correctly due to the fact that SRCU callback invocation accesses the srcu_struct structure's per-CPU data area just after callbacks are invoked. Therefore, the usual practice of using srcu_barrier() to wait for callbacks to be invoked before invoking cleanup_srcu_struct_quiesced() fails because SRCU's callback-invocation workqueue handler might be delayed, which can result in cleanup_srcu_struct_quiesced() being invoked (and thus freeing the per-CPU data) before the SRCU's callback-invocation workqueue handler is finished using that per-CPU data. Nor is this a theoretical problem: KASAN emitted use-after-free warnings because of this problem on actual runs. In short, NVME can now safely invoke cleanup_srcu_struct(), which avoids the use-after-free scenario. And cleanup_srcu_struct_quiesced() is quite difficult to use safely. This commit therefore removes cleanup_srcu_struct_quiesced(), switching its sole user back to cleanup_srcu_struct(). This effectively reverts the following pair of commits: f7194ac32ca2 ("srcu: Add cleanup_srcu_struct_quiesced()") 4317228ad9b8 ("nvme: Avoid flush dependency in delete controller flow") Reported-by: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Reviewed-by: Bart Van Assche <bvanassche@acm.org> Tested-by: Bart Van Assche <bvanassche@acm.org>
2019-03-26srcu: Check for in-flight callbacks in _cleanup_srcu_struct()Paul E. McKenney
If someone fails to drain the corresponding SRCU callbacks (for example, by failing to invoke srcu_barrier()) before invoking either cleanup_srcu_struct() or cleanup_srcu_struct_quiesced(), the resulting diagnostic is an ambiguous use-after-free diagnostic, and even then only if you are running something like KASAN. This commit therefore improves SRCU diagnostics by adding checks for in-flight callbacks at _cleanup_srcu_struct() time. Note that these diagnostics can still be defeated, for example, by invoking call_srcu() concurrently with cleanup_srcu_struct(). Which is a really bad idea, but sometimes all too easy to do. But even then, these diagnostics have at least some probability of catching the problem. Reported-by: Sagi Grimberg <sagi@grimberg.me> Reported-by: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: Bart Van Assche <bvanassche@acm.org>
2019-02-09Merge branches 'doc.2019.01.26a', 'fixes.2019.01.26a', 'sil.2019.01.26a', ↵Paul E. McKenney
'spdx.2019.02.09a', 'srcu.2019.01.26a' and 'torture.2019.01.26a' into HEAD doc.2019.01.26a: Documentation updates. fixes.2019.01.26a: Miscellaneous fixes. sil.2019.01.26a: Removal of a few more spin_is_locked() instances. spdx.2019.02.09a: Add SPDX identifiers to RCU files srcu.2019.01.26a: SRCU updates. torture.2019.01.26a: Torture-test updates.
2019-02-09rcu/srcu: Convert to SPDX license identifierPaul E. McKenney
Replace the license boiler plate with a SPDX license identifier. While in the area, update an email address. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
2019-01-25srcu: Remove srcu_queue_delayed_work_on()Sebastian Andrzej Siewior
srcu_queue_delayed_work_on() disables preemption (and therefore CPU hotplug in RCU's case) and then checks based on its own accounting if a CPU is online. If the CPU is online it uses queue_delayed_work_on() otherwise it fallbacks to queue_delayed_work(). The problem here is that queue_work() on -RT does not work with disabled preemption. queue_work_on() works also on an offlined CPU. queue_delayed_work_on() has the problem that it is possible to program a timer on an offlined CPU. This timer will fire once the CPU is online again. But until then, the timer remains programmed and nothing will happen. Add a local timer which will fire (as requested per delay) on the local CPU and then enqueue the work on the specific CPU. RCUtorture testing with SRCU-P for 24h showed no problems. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2018-11-27srcu: Use "ssp" instead of "sp" for srcu_struct pointerPaul E. McKenney
In RCU, the distinction between "rsp", "rnp", and "rdp" has served well for a great many years, but in SRCU, "sp" vs. "sdp" has proven confusing. This commit therefore renames SRCU's "sp" pointers to "ssp", so that there is "ssp" for srcu_struct pointer, "snp" for srcu_node pointer, and "sdp" for srcu_data pointer. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
2018-11-27srcu: Lock srcu_data structure in srcu_gp_start()Dennis Krein
The srcu_gp_start() function is called with the srcu_struct structure's ->lock held, but not with the srcu_data structure's ->lock. This is problematic because this function accesses and updates the srcu_data structure's ->srcu_cblist, which is protected by that lock. Failing to hold this lock can result in corruption of the SRCU callback lists, which in turn can result in arbitrarily bad results. This commit therefore makes srcu_gp_start() acquire the srcu_data structure's ->lock across the calls to rcu_segcblist_advance() and rcu_segcblist_accelerate(), thus preventing this corruption. Reported-by: Bart Van Assche <bvanassche@acm.org> Reported-by: Christoph Hellwig <hch@infradead.org> Reported-by: Sebastian Kuzminsky <seb.kuzminsky@gmail.com> Signed-off-by: Dennis Krein <Dennis.Krein@netapp.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: Dennis Krein <Dennis.Krein@netapp.com> Cc: <stable@vger.kernel.org> # 4.16.x
2018-11-08srcu: Prevent __call_srcu() counter wrap with read-side critical sectionPaul E. McKenney
Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context"), it has been permissible to use SRCU read-side critical sections in interrupt context. This allows __call_srcu() to use SRCU read-side critical sections to prevent a new SRCU grace period from ending before the call to either srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing SRCU grace-period counter overflow during that time. Note that this does not permit removal of the counter-wrap checks in srcu_gp_end(). These check are necessary to handle the case where a given CPU does not interact at all with SRCU for an extended time period. This commit therefore adds an SRCU read-side critical section to __call_srcu() in order to prevent grace period counter wrap during the funnel-locking process. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-08-30Merge branches 'doc.2018.08.30a', 'dynticks.2018.08.30b', 'srcu.2018.08.30b' ↵Paul E. McKenney
and 'torture.2018.08.29a' into HEAD doc.2018.08.30a: Documentation updates dynticks.2018.08.30b: RCU flavor consolidation updates and cleanups srcu.2018.08.30b: SRCU updates torture.2018.08.29a: Torture-test updates
2018-08-30srcu: Make early-boot call_srcu() reuse workqueue listsPaul E. McKenney
Allocating a list_head structure that is almost never used, and, when used, is used only during early boot (rcu_init() and earlier), is a bit wasteful. This commit therefore eliminates that list_head in favor of the one in the work_struct structure. This is safe because the work_struct structure cannot be used until after rcu_init() returns. Reported-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Tejun Heo <tj@kernel.org> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2018-08-30srcu: Make call_srcu() available during very early bootPaul E. McKenney
Event tracing is moving to SRCU in order to take advantage of the fact that SRCU may be safely used from idle and even offline CPUs. However, event tracing can invoke call_srcu() very early in the boot process, even before workqueue_init_early() is invoked (let alone rcu_init()). Therefore, call_srcu()'s attempts to queue work fail miserably. This commit therefore detects this situation, and refrains from attempting to queue work before rcu_init() time, but does everything else that it would have done, and in addition, adds the srcu_struct to a global list. The rcu_init() function now invokes a new srcu_init() function, which is empty if CONFIG_SRCU=n. Otherwise, srcu_init() queues work for each srcu_struct on the list. This all happens early enough in boot that there is but a single CPU with interrupts disabled, which allows synchronization to be dispensed with. Of course, the queued work won't actually be invoked until after workqueue_init() is invoked, which happens shortly after the scheduler is up and running. This means that although call_srcu() may be invoked any time after per-CPU variables have been set up, there is still a very narrow window when synchronize_srcu() won't work, and this window extends from the time that the scheduler starts until the time that workqueue_init() returns. This can be fixed in a manner similar to the fix for synchronize_rcu_expedited() and friends, but until someone actually needs to use synchronize_srcu() during this window, this fix is added churn for no benefit. Finally, note that Tree SRCU's new srcu_init() function invokes queue_work() rather than the queue_delayed_work() function that is invoked post-boot. The reason is that queue_delayed_work() will (as you would expect) post a timer, and timers have not yet been initialized. So use of queue_work() avoids the complaints about use of uninitialized spinlocks that would otherwise result. Besides, some delay is already provide by the aforementioned fact that the queued work won't actually be invoked until after the scheduler is up and running. Requested-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2018-08-30rcu: Clean up flavor-related definitions and comments in srcutree.hPaul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-08-30rcu: Remove rsp parameter from rcu_node tree accessor macrosPaul E. McKenney
There now is only one rcu_state structure in a given build of the Linux kernel, so there is no need to pass it as a parameter to RCU's rcu_node tree's accessor macros. This commit therefore removes the rsp parameter from those macros in kernel/rcu/rcu.h, and removes some now-unused rsp local variables while in the area. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-07-12srcu: Add grace-period number to rcutorture statistics printoutPaul E. McKenney
This commit adds the SRCU grace-period number to the rcutorture statistics printout, which allows it to be compared to the rcutorture "Writer stall state" message. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-07-12rcu: Use pr_fmt to prefix "rcu: " to logging outputJoe Perches
This commit also adjusts some whitespace while in the area. Signed-off-by: Joe Perches <joe@perches.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> [ paulmck: Revert string-breaking %s as requested by Andy Shevchenko. ]
2018-07-12rcutorture: Convert rcutorture_get_gp_data() to ->gp_seqPaul E. McKenney
SRCU has long used ->srcu_gp_seq, and now RCU uses ->gp_seq. This commit therefore moves the rcutorture_get_gp_data() function from a ->gpnum / ->completed pair to ->gp_seq. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-06-25srcu: Add address of first callback to rcutorture outputPaul E. McKenney
This commit adds the address of the first callback to the per-CPU rcutorture output in order to allow lost wakeups to be more efficiently tracked down. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-06-25srcu: Document that srcu_funnel_gp_start() implies srcu_funnel_exp_start()Paul E. McKenney
This commit updates the header comment of srcu_funnel_gp_start() to document the fact that srcu_funnel_gp_start() does the work of srcu_funnel_exp_start(), in some cases by invoking it directly. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-06-25srcu: Fix typos in __call_srcu() header commentPaul E. McKenney
This commit simply changes some copy-pasta call_rcu() instances to the correct call_srcu(). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-05-15srcu: Add cleanup_srcu_struct_quiesced()Paul E. McKenney
The current cleanup_srcu_struct() flushes work, which prevents it from being invoked from some workqueue contexts, as well as from atomic (non-blocking) contexts. This patch therefore introduced a cleanup_srcu_struct_quiesced(), which can be invoked only after all activity on the specified srcu_struct has completed. This restriction allows cleanup_srcu_struct_quiesced() to be invoked from workqueue contexts as well as from atomic contexts. Suggested-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Nitzan Carmi <nitzanc@mellanox.com> Tested-by: Nicholas Piggin <npiggin@gmail.com>
2018-02-23Merge branches 'fixes.2018.02.23a', 'srcu.2018.02.20a' and ↵Paul E. McKenney
'torture.2018.02.20a' into HEAD fixes.2018.02.23a: Miscellaneous fixes srcu.2018.02.20a: SRCU updates torture.2018.02.20a: Torture-test updates
2018-02-23rcu: Create RCU-specific workqueues with rescuersPaul E. McKenney
RCU's expedited grace periods can participate in out-of-memory deadlocks due to all available system_wq kthreads being blocked and there not being memory available to create more. This commit prevents such deadlocks by allocating an RCU-specific workqueue_struct at early boot time, and providing it with a rescuer to ensure forward progress. This uses the shiny new init_rescuer() function provided by Tejun (but indirectly). This commit also causes SRCU to use this new RCU-specific workqueue_struct. Note that SRCU's use of workqueues never blocks them waiting for readers, so this should be safe from a forward-progress viewpoint. Note that this moves SRCU from system_power_efficient_wq to a normal workqueue. In the unlikely event that this results in measurable degradation, a separate power-efficient workqueue will be creates for SRCU. Reported-by: Prateek Sood <prsood@codeaurora.org> Reported-by: Tejun Heo <tj@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Tejun Heo <tj@kernel.org>
2018-02-20rcu: Remove SRCU throttlingPaul E. McKenney
The code in srcu_gp_end() inserts a delay every 0x3ff grace periods in order to prevent SRCU grace-period work from consuming an entire CPU when there is a long sequence of expedited SRCU grace-period requests. However, all of SRCU's grace-period work is carried out in workqueues, which are in turn within kthreads, which are automatically throttled as needed by the scheduler. In particular, if there is plenty of idle time, there is no point in throttling. This commit therefore removes the expedited SRCU grace-period throttling. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20srcu: Remove dead code in srcu_gp_end()Byungchul Park
Of course, compilers will optimize out a dead code. Anyway, remove any dead code for better readibility. Signed-off-by: Byungchul Park <byungchul.park@lge.com> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20srcu: Reduce scans of srcu_data in counter wrap checkIldar Ismagilov
Currently, given a multi-level srcu_node tree, SRCU can scan the full set of srcu_data structures at each level when cleaning up after a grace period. This, though harmless otherwise, represents pointless overhead. This commit therefore eliminates this overhead by scanning the srcu_data structures only when traversing the leaf srcu_node structures. Signed-off-by: Ildar Ismagilov <devix84@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20srcu: Prevent sdp->srcu_gp_seq_needed_exp counter wrapIldar Ismagilov
SRCU checks each srcu_data structure's grace-period number for counter wrap four times per cycle by default. This frequency guarantees that normal comparisons will detect potential wrap. However, the expedited grace-period number is not checked. The consquences are not too horrible (a failure to expedite a grace period when requested), but it would be good to avoid such things. This commit therefore adds this check to the expedited grace-period number. Signed-off-by: Ildar Ismagilov <devix84@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20srcu: Abstract function namePaul E. McKenney
This commit moves to __func__ for function names in the name of better resilience to change. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20rcu: Fix misprint in srcu_funnel_exp_startIldar Ismagilov
The srcu_funnel_exp_start() function checks to see if the srcu_struct structure's expedited grace period counter needs updating to reflect a newly arrived request for an expedited SRCU grace period. Unfortunately, the check is backwards, so this commit reverses the sense of the test. Signed-off-by: Ildar Ismagilov <devix84@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20rcu: Use wrapper for lockdep assertsMatthew Wilcox
Commits c0b334c5bfa9 and ea9b0c8a26a2 introduced new sparse warnings by accessing rcu_node->lock directly and ignoring the __private marker. Introduce a new wrapper and use it. Also fix a similar problem in srcutree.c introduced by a3883df3935e. Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28srcu: Prohibit call_srcu() use under raw spinlocksPaul E. McKenney
Invoking queue_delayed_work() while holding a raw spinlock is forbidden in -rt kernels, which is exactly what __call_srcu() does, indirectly via srcu_funnel_gp_start(). This commit therefore downgrades Tree SRCU's locking from raw to non-raw spinlocks, which works because call_srcu() is not ever called while holding a raw spinlock. Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-10-19doc: Fix various RCU docbook comment-header problemsPaul E. McKenney
Because many of RCU's files have not been included into docbook, a number of errors have accumulated. This commit fixes them. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-08-17Merge branches 'doc.2017.08.17a', 'fixes.2017.08.17a', ↵Paul E. McKenney
'hotplug.2017.07.25b', 'misc.2017.08.17a', 'spin_unlock_wait_no.2017.08.17a', 'srcu.2017.07.27c' and 'torture.2017.07.24c' into HEAD doc.2017.08.17a: Documentation updates. fixes.2017.08.17a: RCU fixes. hotplug.2017.07.25b: CPU-hotplug updates. misc.2017.08.17a: Miscellaneous fixes outside of RCU (give or take conflicts). spin_unlock_wait_no.2017.08.17a: Remove spin_unlock_wait(). srcu.2017.07.27c: SRCU updates. torture.2017.07.24c: Torture-test updates.
2017-07-27srcu: Provide ordering for CPU not involved in grace periodPaul E. McKenney
Tree RCU guarantees that every online CPU has a memory barrier between any given grace period and any of that CPU's RCU read-side sections that must be ordered against that grace period. Since RCU doesn't always know where read-side critical sections are, the actual implementation guarantees order against prior and subsequent non-idle non-offline code, whether in an RCU read-side critical section or not. As a result, there does not need to be a memory barrier at the end of synchronize_rcu() and friends because the ordering internal to the grace period has ordered every CPU's post-grace-period execution against each CPU's pre-grace-period execution, again for all non-idle online CPUs. In contrast, SRCU can have non-idle online CPUs that are completely uninvolved in a given SRCU grace period, for example, a CPU that never runs any SRCU read-side critical sections and took no part in the grace-period processing. It is in theory possible for a given synchronize_srcu()'s wakeup to be delivered to a CPU that was completely uninvolved in the prior SRCU grace period, which could mean that the code following that synchronize_srcu() would end up being unordered with respect to both the grace period and any pre-existing SRCU read-side critical sections. This commit therefore adds an smp_mb() to the end of __synchronize_srcu(), which prevents this scenario from occurring. Reported-by: Lance Roy <ldr709@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Lance Roy <ldr709@gmail.com> Cc: <stable@vger.kernel.org> # 4.12.x
2017-07-24rcutorture: Print SRCU lock/unlock totalsPaul E. McKenney
This commit adds printing of SRCU lock/unlock totals, which are just the sums of the per-CPU counts. Saves a bit of mental arithmetic. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-24rcutorture: Move SRCU status printing to SRCU implementationsPaul E. McKenney
This commit gets rid of some ugly #ifdefs in rcutorture.c by moving the SRCU status printing to the SRCU implementations. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-24srcu: Make process_srcu() be staticPaul E. McKenney
The function process_srcu() is not invoked outside of srcutree.c, so this commit makes it static and drops the EXPORT_SYMBOL_GPL(). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Use rnp->lock wrappers to replace explicit memory barriersPaul E. McKenney
This commit uses TREE RCU's rnp->lock wrappers to replace a few explicit memory barriers. This change also has the advantage of making SRCU's memory-ordering properties be implemented in roughly the same way as they are in Tree RCU. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Shrink srcu.h by moving docbook and private functionPaul E. McKenney
The call_srcu() docbook entry is currently in include/linux/srcu.h, which causes needless processing for each include point. This commit therefore moves this entry to kernel/rcu/srcutree.c, which the compiler reads only once. In addition, the srcu_batches_completed() function is used only within RCU and its torture-test suites. This commit therefore also moves this function's declaration from include/linux/srcutiny.h, include/linux/srcutree.h, and include/linux/srcuclassic.h to kernel/rcu/rcu.h. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Prevent sdp->srcu_gp_seq_needed counter wrapPaul E. McKenney
If a given CPU never happens to ever start an SRCU grace period, the grace-period sequence counter might wrap. If this CPU were to decide to finally start a grace period, the state of its sdp->srcu_gp_seq_needed might make it appear that it has already requested this grace period, which would prevent starting the grace period. If no other CPU ever started a grace period again, this would look like a grace-period hang. Even if some other CPU took pity and started the needed grace period, the leaf rcu_node structure's ->srcu_data_have_cbs field won't have record of the fact that this CPU has a callback pending, which would look like a very localized grace-period hang. This might seem very unlikely, but SRCU grace periods can take less than a microsecond on small systems, which means that overflow can happen in much less than an hour on a 32-bit embedded system. And embedded systems are especially likely to have long-term idle CPUs. Therefore, it makes sense to prevent this scenario from happening. This commit therefore scans each srcu_data structure occasionally, with frequency controlled by the srcutree.counter_wrap_check kernel boot parameter. This parameter can be set to something like 255 in order to exercise the counter-wrap-prevention code. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Add DEBUG_OBJECTS_RCU_HEAD functionalityPaul E. McKenney
This commit adds DEBUG_OBJECTS_RCU_HEAD checking to detect call_srcu() counterparts to double-free bugs. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Print non-default exp_holdoff values at boot timePaul E. McKenney
This commit makes srcu_bootup_announce() check for non-default values of the auto-expedite holdoff time exp_holdoff and print a message if so. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Make exp_holdoff module parameter be staticPaul E. McKenney
Because exp_holdoff is not used outside of srcutree.c, it can be static. This commit therefore makes this change. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Make Classic and Tree SRCU announce themselves at bootupPaul E. McKenney
Currently, the only way to tell whether a given kernel is running Classic, Tiny, or Tree SRCU is to look at the .config file, which can easily be lost or associated with the wrong kernel. This commit therefore has Classic and Tree SRCU identify themselves at boot time. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Eliminate possibility of destructive counter overflowPaul E. McKenney
Earlier versions of Tree SRCU were subject to a counter overflow bug that could theoretically result in too-short grace periods. This commit eliminates this problem by adding an update-side memory barrier. The short explanation is that if the updater sums the unlock counts too late to see a given __srcu_read_unlock() increment, that CPU's next __srcu_read_lock() must see the new value of ->srcu_idx, thus incrementing the other bank of counters. This eliminates the possibility of destructive counter overflow as long as the srcu_read_lock() nesting level does not exceed floor(ULONG_MAX/NR_CPUS/2), which should be an eminently reasonable nesting limit, especially on 64-bit systems. Reported-by: Lance Roy <ldr709@gmail.com> Suggested-by: Lance Roy <ldr709@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08srcu: Allow use of Tiny/Tree SRCU from both process and interrupt contextPaolo Bonzini
Linu Cherian reported a WARN in cleanup_srcu_struct() when shutting down a guest running iperf on a VFIO assigned device. This happens because irqfd_wakeup() calls srcu_read_lock(&kvm->irq_srcu) in interrupt context, while a worker thread does the same inside kvm_set_irq(). If the interrupt happens while the worker thread is executing __srcu_read_lock(), updates to the Classic SRCU ->lock_count[] field or the Tree SRCU ->srcu_lock_count[] field can be lost. The docs say you are not supposed to call srcu_read_lock() and srcu_read_unlock() from irq context, but KVM interrupt injection happens from (host) interrupt context and it would be nice if SRCU supported the use case. KVM is using SRCU here not really for the "sleepable" part, but rather due to its IPI-free fast detection of grace periods. It is therefore not desirable to switch back to RCU, which would effectively revert commit 719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING", 2014-01-16). However, the docs are overly conservative. You can have an SRCU instance only has users in irq context, and you can mix process and irq context as long as process context users disable interrupts. In addition, __srcu_read_unlock() actually uses this_cpu_dec() on both Tree SRCU and Classic SRCU. For those two implementations, only srcu_read_lock() is unsafe. When Classic SRCU's __srcu_read_unlock() was changed to use this_cpu_dec(), in commit 5a41344a3d83 ("srcu: Simplify __srcu_read_unlock() via this_cpu_dec()", 2012-11-29), __srcu_read_lock() did two increments. Therefore it kept __this_cpu_inc(), with preempt_disable/enable in the caller. Tree SRCU however only does one increment, so on most architectures it is more efficient for __srcu_read_lock() to use this_cpu_inc(), and any performance differences appear to be down in the noise. Unlike Classic and Tree SRCU, Tiny SRCU does increments and decrements on a single variable. Therefore, as Peter Zijlstra pointed out, Tiny SRCU's implementation already supports mixed-context use of srcu_read_lock() and srcu_read_unlock(), at least as long as uses of srcu_read_lock() and srcu_read_unlock() in each handler are nested and paired properly. In other words, it is still illegal to (say) invoke srcu_read_lock() in an interrupt handler and to invoke the matching srcu_read_unlock() in a softirq handler. Therefore, the only change required for Tiny SRCU is to its comments. Fixes: 719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING") Reported-by: Linu Cherian <linuc.decode@gmail.com> Suggested-by: Linu Cherian <linuc.decode@gmail.com> Cc: kvm@vger.kernel.org Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Paolo Bonzini <pbonzini@redhat.com>
2017-05-02srcu: Debloat the <linux/rcu_segcblist.h> headerIngo Molnar
Linus noticed that the <linux/rcu_segcblist.h> has huge inline functions which should not be inline at all. As a first step in cleaning this up, move them all to kernel/rcu/ and only keep an absolute minimum of data type defines in the header: before: -rw-r--r-- 1 mingo mingo 22284 May 2 10:25 include/linux/rcu_segcblist.h after: -rw-r--r-- 1 mingo mingo 3180 May 2 10:22 include/linux/rcu_segcblist.h More can be done, such as uninlining the large functions, which inlining is unjustified even if it's an RCU internal matter. Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-27srcu: Adjust default auto-expediting holdoffPaul E. McKenney
The default value for the kernel boot parameter srcutree.exp_holdoff is 50 microseconds, which is too long for good Tree SRCU performance (compared to Classic SRCU) on the workloads tested by Mike Galbraith. This commit therefore sets the default value to 25 microseconds, which shows excellent results in Mike's testing. Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Mike Galbraith <efault@gmx.de>
2017-04-26srcu: Specify auto-expedite holdoff timePaul E. McKenney
On small systems, in the absence of readers, expedited SRCU grace periods can complete in less than a microsecond. This means that an eight-CPU system can have all CPUs doing synchronize_srcu() in a tight loop and almost always expedite. This might actually be desirable in some situations, but in general it is a good way to needlessly burn CPU cycles. And in those situations where it is desirable, your friend is the function synchronize_srcu_expedited(). For other situations, this commit adds a kernel parameter that specifies a holdoff between completing the last SRCU grace period and auto-expediting the next. If the next grace period starts before the holdoff expires, auto-expediting is disabled. The holdoff is 50 microseconds by default, and can be tuned to the desired number of nanoseconds. A value of zero disables auto-expediting. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Mike Galbraith <efault@gmx.de>