Commit fd638368 authored Dec 25, 2011 by Tejun Heo Committed by Jens Axboe Dec 25, 2011

block: an exiting task should be allowed to create io_context



While fixing io_context creation / task exit race condition,
6e736be7 "block: make ioc get/put interface more conventional and
fix race on alloction" also prevented an exiting (%PF_EXITING) task
from creating its own io_context.  This is incorrect as exit path may
issue IOs, e.g. from exit_files(), and if those IOs are the first ones
issued by the task, io_context needs to be created to process the IOs.

Combined with the existing problem of io_context / io_cq creation
failure having the possibility of stalling IO, this problem results in
deterministic full IO lockup with certain workloads.

Fix it by allowing io_context creation regardless of %PF_EXITING for
%current.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 64c42998

block/blk-ioc.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -281,9 +281,16 @@ void create_io_context_slowpath(struct task_struct *task, gfp_t gfp_flags,
		INIT_HLIST_HEAD(&ioc->icq_list);
		INIT_WORK(&ioc->release_work, ioc_release_fn);

		/* try to install, somebody might already have beaten us to it */
		/*
		* Try to install. ioc shouldn't be installed if someone else
		* already did or @task, which isn't %current, is exiting. Note
		* that we need to allow ioc creation on exiting %current as exit
		* path may issue IOs from e.g. exit_files(). The exit path is
		* responsible for not issuing IO after exit_io_context().
		*/
		task_lock(task);
		if (!task->io_context && !(task->flags & PF_EXITING))
		if (!task->io_context &&
		(task == current \|\| !(task->flags & PF_EXITING)))
		task->io_context = ioc;
		else
		kmem_cache_free(iocontext_cachep, ioc);

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