Hello, Found the culprit. Plain wake_up() shouldn't be used on bit_waitqueues. The following patch should fix the issue. I replaced the patch in the wq branches.
Thanks a lot. ----- 8< ----- >From bba5a377507efef69214108c0d3790cadfab21d4 Mon Sep 17 00:00:00 2001 From: Tejun Heo <[email protected]> Date: Tue, 3 Mar 2015 08:43:09 -0500 cancel[_delayed]_work_sync() are implemented using __cancel_work_timer() which grabs the PENDING bit using try_to_grab_pending() and then flushes the work item with PENDING set to prevent the on-going execution of the work item from requeueing itself. try_to_grab_pending() can always grab PENDING bit without blocking except when someone else is doing the above flushing during cancelation. In that case, try_to_grab_pending() returns -ENOENT. In this case, __cancel_work_timer() currently invokes flush_work(). The assumption is that the completion of the work item is what the other canceling task would be waiting for too and thus waiting for the same condition and retrying should allow forward progress without excessive busy looping Unfortunately, this doesn't work if preemption is disabled or the latter task has real time priority. Let's say task A just got woken up from flush_work() by the completion of the target work item. If, before task A starts executing, task B gets scheduled and invokes __cancel_work_timer() on the same work item, its try_to_grab_pending() will return -ENOENT as the work item is still being canceled by task A and flush_work() will also immediately return false as the work item is no longer executing. This puts task B in a busy loop possibly preventing task A from executing and clearing the canceling state on the work item leading to a hang. task A task B worker executing work __cancel_work_timer() try_to_grab_pending() set work CANCELING flush_work() block for work completion completion, wakes up A __cancel_work_timer() while (forever) { try_to_grab_pending() -ENOENT as work is being canceled flush_work() false as work is no longer executing } This patch removes the possible hang by updating __cancel_work_timer() to explicitly wait for clearing of CANCELING rather than invoking flush_work() after try_to_grab_pending() fails with -ENOENT. The explicit wait uses the matching bit waitqueue for the CANCELING bit. Link: http://lkml.kernel.org/g/[email protected] v2: v1 used wake_up() on bit_waitqueue() which leads to NULL deref if the target bit waitqueue has wait_bit_queue's on it. Use DEFINE_WAIT_BIT() and __wake_up_bit() instead. Reported by Tomeu Vizoso. Signed-off-by: Tejun Heo <[email protected]> Reported-by: Rabin Vincent <[email protected]> Cc: Tomeu Vizoso <[email protected]> Cc: [email protected] Tested-by: Jesper Nilsson <[email protected]> Tested-by: Rabin Vincent <[email protected]> --- include/linux/workqueue.h | 3 ++- kernel/workqueue.c | 37 +++++++++++++++++++++++++++++++++---- 2 files changed, 35 insertions(+), 5 deletions(-) diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h index 74db135..f597846 100644 --- a/include/linux/workqueue.h +++ b/include/linux/workqueue.h @@ -70,7 +70,8 @@ enum { /* data contains off-queue information when !WORK_STRUCT_PWQ */ WORK_OFFQ_FLAG_BASE = WORK_STRUCT_COLOR_SHIFT, - WORK_OFFQ_CANCELING = (1 << WORK_OFFQ_FLAG_BASE), + __WORK_OFFQ_CANCELING = WORK_OFFQ_FLAG_BASE, + WORK_OFFQ_CANCELING = (1 << __WORK_OFFQ_CANCELING), /* * When a work item is off queue, its high bits point to the last diff --git a/kernel/workqueue.c b/kernel/workqueue.c index f288493..cfbae1d 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -2730,17 +2730,37 @@ EXPORT_SYMBOL_GPL(flush_work); static bool __cancel_work_timer(struct work_struct *work, bool is_dwork) { + wait_queue_head_t *waitq = bit_waitqueue(&work->data, + __WORK_OFFQ_CANCELING); unsigned long flags; int ret; do { ret = try_to_grab_pending(work, is_dwork, &flags); /* - * If someone else is canceling, wait for the same event it - * would be waiting for before retrying. + * If someone else is already canceling, wait for it to + * finish. flush_work() doesn't work for PREEMPT_NONE + * because we may get scheduled between @work's completion + * and the other canceling task resuming and clearing + * CANCELING - flush_work() will return false immediately + * as @work is no longer busy, try_to_grab_pending() will + * return -ENOENT as @work is still being canceled and the + * other canceling task won't be able to clear CANCELING as + * we're hogging the CPU. + * + * Explicitly wait for completion using a bit waitqueue. + * We can't use wait_on_bit() as the CANCELING bit may get + * recycled to point to pwq if @work gets re-queued. */ - if (unlikely(ret == -ENOENT)) - flush_work(work); + if (unlikely(ret == -ENOENT)) { + DEFINE_WAIT_BIT(wait, &work->data, + __WORK_OFFQ_CANCELING); + prepare_to_wait(waitq, &wait.wait, + TASK_UNINTERRUPTIBLE); + if (work_is_canceling(work)) + schedule(); + finish_wait(waitq, &wait.wait); + } } while (unlikely(ret < 0)); /* tell other tasks trying to grab @work to back off */ @@ -2749,6 +2769,15 @@ static bool __cancel_work_timer(struct work_struct *work, bool is_dwork) flush_work(work); clear_work_data(work); + + /* + * Paired with prepare_to_wait() above so that either + * __wake_up_bit() sees busy waitq here or !work_is_canceling() is + * visible there. + */ + smp_mb(); + __wake_up_bit(waitq, &work->data, __WORK_OFFQ_CANCELING); + return ret; } -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
