While fixing io_context creation / task exit race condition, 6e736be7f2 "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@xxxxxxxxxx> Reported-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx> --- Thanks a lot for the hint, Hugh. My testing stuff (fio, dd and some adhoc rawio testing programs) was issuing IOs before exiting, so I didn't hit the problem and I suspect the reason why I didn't see the boot failure Andrew was seeing was because of systemd - boot process used to be dominated by lots of short-lived programs, many of which touching/modifying files, and thus it triggered the first IO on exit paths with Andrew's old userland. With systemd, most of those are gone, so... Thanks. block/blk-ioc.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/block/blk-ioc.c b/block/blk-ioc.c index ce9b35a..33fae7d 100644 --- a/block/blk-ioc.c +++ b/block/blk-ioc.c @@ -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); -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
