Tejun Heo d8bee0e3ab workqueue: fix hang involving racing cancel[_delayed]_work_sync()'s for PREEMPT_NONE ... commit 8603e1b30027f943cc9c1eef2b291d42c3347af1 upstream. 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. Link: http://lkml.kernel.org/g/20150206171156.GA8942@axis.com v3: bit_waitqueue() can't be used for work items defined in vmalloc area. Switched to custom wake function which matches the target work item and exclusive wait and wakeup. 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 <tj@kernel.org> Reported-by: Rabin Vincent <rabin.vincent@axis.com> Cc: Tomeu Vizoso <tomeu.vizoso@gmail.com> Tested-by: Jesper Nilsson <jesper.nilsson@axis.com> Tested-by: Rabin Vincent <rabin.vincent@axis.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2015-03-26 15:00:58 +01:00
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acpi
asm-generic
clocksource
crypto
drm	drm/radeon: remove invalid pci id	2014-11-14 08:48:00 -08:00
dt-bindings
keys
linux	workqueue: fix hang involving racing cancel[_delayed]_work_sync()'s for PREEMPT_NONE	2015-03-26 15:00:58 +01:00
math-emu
media
memory
misc
net	ipv4: tcp: get rid of ugly unicast_sock	2015-02-26 17:48:48 -08:00
pcmcia
ras
rdma
rxrpc
scsi
sound	ALSA: ak411x: Fix stall in work callback	2015-02-11 14:48:17 +08:00
target
trace
uapi	netfilter: xt_bpf: add mising opaque struct sk_filter definition	2014-11-21 09:22:54 -08:00
video
xen
Kbuild