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Diffstat (limited to 'Documentation/locking/locktypes.rst')
| -rw-r--r-- | Documentation/locking/locktypes.rst | 148 |
1 files changed, 98 insertions, 50 deletions
diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst index f0aa91142e94..1c18bb8785e2 100644 --- a/Documentation/locking/locktypes.rst +++ b/Documentation/locking/locktypes.rst @@ -67,6 +67,17 @@ can have suffixes which apply further protections: _irqsave/restore() Save and disable / restore interrupt disabled state =================== ==================================================== +Owner semantics +=============== + +The aforementioned lock types except semaphores have strict owner +semantics: + + The context (task) that acquired the lock must release it. + +rw_semaphores have a special interface which allows non-owner release for +readers. + rtmutex ======= @@ -83,6 +94,51 @@ interrupt handlers and soft interrupts. This conversion allows spinlock_t and rwlock_t to be implemented via RT-mutexes. +semaphore +========= + +semaphore is a counting semaphore implementation. + +Semaphores are often used for both serialization and waiting, but new use +cases should instead use separate serialization and wait mechanisms, such +as mutexes and completions. + +semaphores and PREEMPT_RT +---------------------------- + +PREEMPT_RT does not change the semaphore implementation because counting +semaphores have no concept of owners, thus preventing PREEMPT_RT from +providing priority inheritance for semaphores. After all, an unknown +owner cannot be boosted. As a consequence, blocking on semaphores can +result in priority inversion. + + +rw_semaphore +============ + +rw_semaphore is a multiple readers and single writer lock mechanism. + +On non-PREEMPT_RT kernels the implementation is fair, thus preventing +writer starvation. + +rw_semaphore complies by default with the strict owner semantics, but there +exist special-purpose interfaces that allow non-owner release for readers. +These interfaces work independent of the kernel configuration. + +rw_semaphore and PREEMPT_RT +--------------------------- + +PREEMPT_RT kernels map rw_semaphore to a separate rt_mutex-based +implementation, thus changing the fairness: + + Because an rw_semaphore writer cannot grant its priority to multiple + readers, a preempted low-priority reader will continue holding its lock, + thus starving even high-priority writers. In contrast, because readers + can grant their priority to a writer, a preempted low-priority writer will + have its priority boosted until it releases the lock, thus preventing that + writer from starving readers. + + raw_spinlock_t and spinlock_t ============================= @@ -102,7 +158,7 @@ critical section is tiny, thus avoiding RT-mutex overhead. spinlock_t ---------- -The semantics of spinlock_t change with the state of CONFIG_PREEMPT_RT. +The semantics of spinlock_t change with the state of PREEMPT_RT. On a non PREEMPT_RT enabled kernel spinlock_t is mapped to raw_spinlock_t and has exactly the same semantics. @@ -140,7 +196,16 @@ PREEMPT_RT kernels preserve all other spinlock_t semantics: kernels leave task state untouched. However, PREEMPT_RT must change task state if the task blocks during acquisition. Therefore, it saves the current task state before blocking and the corresponding lock wakeup - restores it. + restores it, as shown below:: + + task->state = TASK_INTERRUPTIBLE + lock() + block() + task->saved_state = task->state + task->state = TASK_UNINTERRUPTIBLE + schedule() + lock wakeup + task->state = task->saved_state Other types of wakeups would normally unconditionally set the task state to RUNNING, but that does not work here because the task must remain @@ -148,7 +213,22 @@ PREEMPT_RT kernels preserve all other spinlock_t semantics: wakeup attempts to awaken a task blocked waiting for a spinlock, it instead sets the saved state to RUNNING. Then, when the lock acquisition completes, the lock wakeup sets the task state to the saved - state, in this case setting it to RUNNING. + state, in this case setting it to RUNNING:: + + task->state = TASK_INTERRUPTIBLE + lock() + block() + task->saved_state = task->state + task->state = TASK_UNINTERRUPTIBLE + schedule() + non lock wakeup + task->saved_state = TASK_RUNNING + + lock wakeup + task->state = task->saved_state + + This ensures that the real wakeup cannot be lost. + rwlock_t ======== @@ -228,17 +308,16 @@ preemption on PREEMPT_RT kernels:: bit spinlocks ------------- -Bit spinlocks are problematic for PREEMPT_RT as they cannot be easily -substituted by an RT-mutex based implementation for obvious reasons. - -The semantics of bit spinlocks are preserved on PREEMPT_RT kernels and the -caveats vs. raw_spinlock_t apply. +PREEMPT_RT cannot substitute bit spinlocks because a single bit is too +small to accommodate an RT-mutex. Therefore, the semantics of bit +spinlocks are preserved on PREEMPT_RT kernels, so that the raw_spinlock_t +caveats also apply to bit spinlocks. -Some bit spinlocks are substituted by regular spinlock_t for PREEMPT_RT but -this requires conditional (#ifdef'ed) code changes at the usage site while -the spinlock_t substitution is simply done by the compiler and the -conditionals are restricted to header files and core implementation of the -locking primitives and the usage sites do not require any changes. +Some bit spinlocks are replaced with regular spinlock_t for PREEMPT_RT +using conditional (#ifdef'ed) code changes at the usage site. In contrast, +usage-site changes are not needed for the spinlock_t substitution. +Instead, conditionals in header files and the core locking implemementation +enable the compiler to do the substitution transparently. Lock type nesting rules @@ -254,46 +333,15 @@ The most basic rules are: - Spinning lock types can nest inside sleeping lock types. -These rules apply in general independent of CONFIG_PREEMPT_RT. +These constraints apply both in PREEMPT_RT and otherwise. -As PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from -spinning to sleeping this has obviously restrictions how they can nest with -raw_spinlock_t. - -This results in the following nest ordering: +The fact that PREEMPT_RT changes the lock category of spinlock_t and +rwlock_t from spinning to sleeping means that they cannot be acquired while +holding a raw spinlock. This results in the following nesting ordering: 1) Sleeping locks 2) spinlock_t and rwlock_t 3) raw_spinlock_t and bit spinlocks -Lockdep is aware of these constraints to ensure that they are respected. - - -Owner semantics -=============== - -Most lock types in the Linux kernel have strict owner semantics, i.e. the -context (task) which acquires a lock has to release it. - -There are two exceptions: - - - semaphores - - rwsems - -semaphores have no owner semantics for historical reason, and as such -trylock and release operations can be called from any context. They are -often used for both serialization and waiting purposes. That's generally -discouraged and should be replaced by separate serialization and wait -mechanisms, such as mutexes and completions. - -rwsems have grown interfaces which allow non owner release for special -purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT -substitutes all locking primitives except semaphores with RT-mutex based -implementations to provide priority inheritance for all lock types except -the truly spinning ones. Priority inheritance on ownerless locks is -obviously impossible. - -For now the rwsem non-owner release excludes code which utilizes it from -being used on PREEMPT_RT enabled kernels. In same cases this can be -mitigated by disabling portions of the code, in other cases the complete -functionality has to be disabled until a workable solution has been found. +Lockdep will complain if these constraints are violated, both in +PREEMPT_RT and otherwise. |