From 6e84d644b5929789398914b0ccf447355dec6fb0 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 9 May 2007 02:34:46 -0700 Subject: make cancel_rearming_delayed_work() reliable Thanks to Jarek Poplawski for the ideas and for spotting the bug in the initial draft patch. cancel_rearming_delayed_work() currently has many limitations, because it requires that dwork always re-arms itself via queue_delayed_work(). So it hangs forever if dwork doesn't do this, or cancel_rearming_delayed_work/ cancel_delayed_work was already called. It uses flush_workqueue() in a loop, so it can't be used if workqueue was freezed, and it is potentially live- lockable on busy system if delay is small. With this patch cancel_rearming_delayed_work() doesn't make any assumptions about dwork, it can re-arm itself via queue_delayed_work(), or queue_work(), or do nothing. As a "side effect", cancel_work_sync() was changed to handle re-arming works as well. Disadvantages: - this patch adds wmb() to insert_work(). - slowdowns the fast path (when del_timer() succeeds on entry) of cancel_rearming_delayed_work(), because wait_on_work() is called unconditionally. In that case, compared to the old version, we are doing "unneeded" lock/unlock for each online CPU. On the other hand, this means we don't need to use cancel_work_sync() after cancel_rearming_delayed_work(). - complicates the code (.text grows by 130 bytes). [akpm@linux-foundation.org: fix speling] Signed-off-by: Oleg Nesterov Cc: David Chinner Cc: David Howells Cc: Gautham Shenoy Acked-by: Jarek Poplawski Cc: Srivatsa Vaddagiri Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/workqueue.c | 140 ++++++++++++++++++++++++++++++++++------------------- 1 file changed, 91 insertions(+), 49 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 25cee1afe6fb..b976ed87dd37 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -120,6 +120,11 @@ static void insert_work(struct cpu_workqueue_struct *cwq, struct work_struct *work, int tail) { set_wq_data(work, cwq); + /* + * Ensure that we get the right work->data if we see the + * result of list_add() below, see try_to_grab_pending(). + */ + smp_wmb(); if (tail) list_add_tail(&work->entry, &cwq->worklist); else @@ -383,7 +388,46 @@ void fastcall flush_workqueue(struct workqueue_struct *wq) } EXPORT_SYMBOL_GPL(flush_workqueue); -static void wait_on_work(struct cpu_workqueue_struct *cwq, +/* + * Upon a successful return, the caller "owns" WORK_STRUCT_PENDING bit, + * so this work can't be re-armed in any way. + */ +static int try_to_grab_pending(struct work_struct *work) +{ + struct cpu_workqueue_struct *cwq; + int ret = 0; + + if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work))) + return 1; + + /* + * The queueing is in progress, or it is already queued. Try to + * steal it from ->worklist without clearing WORK_STRUCT_PENDING. + */ + + cwq = get_wq_data(work); + if (!cwq) + return ret; + + spin_lock_irq(&cwq->lock); + if (!list_empty(&work->entry)) { + /* + * This work is queued, but perhaps we locked the wrong cwq. + * In that case we must see the new value after rmb(), see + * insert_work()->wmb(). + */ + smp_rmb(); + if (cwq == get_wq_data(work)) { + list_del_init(&work->entry); + ret = 1; + } + } + spin_unlock_irq(&cwq->lock); + + return ret; +} + +static void wait_on_cpu_work(struct cpu_workqueue_struct *cwq, struct work_struct *work) { struct wq_barrier barr; @@ -400,20 +444,7 @@ static void wait_on_work(struct cpu_workqueue_struct *cwq, wait_for_completion(&barr.done); } -/** - * cancel_work_sync - block until a work_struct's callback has terminated - * @work: the work which is to be flushed - * - * cancel_work_sync() will attempt to cancel the work if it is queued. If the - * work's callback appears to be running, cancel_work_sync() will block until - * it has completed. - * - * cancel_work_sync() is designed to be used when the caller is tearing down - * data structures which the callback function operates upon. It is expected - * that, prior to calling cancel_work_sync(), the caller has arranged for the - * work to not be requeued. - */ -void cancel_work_sync(struct work_struct *work) +static void wait_on_work(struct work_struct *work) { struct cpu_workqueue_struct *cwq; struct workqueue_struct *wq; @@ -423,29 +454,62 @@ void cancel_work_sync(struct work_struct *work) might_sleep(); cwq = get_wq_data(work); - /* Was it ever queued ? */ if (!cwq) return; - /* - * This work can't be re-queued, no need to re-check that - * get_wq_data() is still the same when we take cwq->lock. - */ - spin_lock_irq(&cwq->lock); - list_del_init(&work->entry); - work_clear_pending(work); - spin_unlock_irq(&cwq->lock); - wq = cwq->wq; cpu_map = wq_cpu_map(wq); for_each_cpu_mask(cpu, *cpu_map) - wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work); + wait_on_cpu_work(per_cpu_ptr(wq->cpu_wq, cpu), work); +} + +/** + * cancel_work_sync - block until a work_struct's callback has terminated + * @work: the work which is to be flushed + * + * cancel_work_sync() will cancel the work if it is queued. If the work's + * callback appears to be running, cancel_work_sync() will block until it + * has completed. + * + * It is possible to use this function if the work re-queues itself. It can + * cancel the work even if it migrates to another workqueue, however in that + * case it only guarantees that work->func() has completed on the last queued + * workqueue. + * + * cancel_work_sync(&delayed_work->work) should be used only if ->timer is not + * pending, otherwise it goes into a busy-wait loop until the timer expires. + * + * The caller must ensure that workqueue_struct on which this work was last + * queued can't be destroyed before this function returns. + */ +void cancel_work_sync(struct work_struct *work) +{ + while (!try_to_grab_pending(work)) + cpu_relax(); + wait_on_work(work); + work_clear_pending(work); } EXPORT_SYMBOL_GPL(cancel_work_sync); +/** + * cancel_rearming_delayed_work - reliably kill off a delayed work. + * @dwork: the delayed work struct + * + * It is possible to use this function if @dwork rearms itself via queue_work() + * or queue_delayed_work(). See also the comment for cancel_work_sync(). + */ +void cancel_rearming_delayed_work(struct delayed_work *dwork) +{ + while (!del_timer(&dwork->timer) && + !try_to_grab_pending(&dwork->work)) + cpu_relax(); + wait_on_work(&dwork->work); + work_clear_pending(&dwork->work); +} +EXPORT_SYMBOL(cancel_rearming_delayed_work); -static struct workqueue_struct *keventd_wq; +static struct workqueue_struct *keventd_wq __read_mostly; /** * schedule_work - put work task in global workqueue @@ -531,28 +595,6 @@ void flush_scheduled_work(void) } EXPORT_SYMBOL(flush_scheduled_work); -/** - * cancel_rearming_delayed_work - kill off a delayed work whose handler rearms the delayed work. - * @dwork: the delayed work struct - * - * Note that the work callback function may still be running on return from - * cancel_delayed_work(). Run flush_workqueue() or cancel_work_sync() to wait - * on it. - */ -void cancel_rearming_delayed_work(struct delayed_work *dwork) -{ - struct cpu_workqueue_struct *cwq = get_wq_data(&dwork->work); - - /* Was it ever queued ? */ - if (cwq != NULL) { - struct workqueue_struct *wq = cwq->wq; - - while (!cancel_delayed_work(dwork)) - flush_workqueue(wq); - } -} -EXPORT_SYMBOL(cancel_rearming_delayed_work); - /** * execute_in_process_context - reliably execute the routine with user context * @fn: the function to execute -- cgit v1.2.3