The patch titled Subject: kthread: fix kthread_mod_delayed_work vs kthread_cancel_delayed_work_sync race has been removed from the -mm tree. Its filename was kthread-fix-kthread_mod_delayed_work-vs-kthread_cancel_delayed_work_sync-race.patch

This patch was dropped because an updated version will be merged

------------------------------------------------------ From: Martin Liu liumartin@google.com Subject: kthread: fix kthread_mod_delayed_work vs kthread_cancel_delayed_work_sync race

We encountered a system hang issue while doing the tests. The callstack is as following

schedule+0x80/0x100 schedule_timeout+0x48/0x138 wait_for_common+0xa4/0x134 wait_for_completion+0x1c/0x2c kthread_flush_work+0x114/0x1cc kthread_cancel_work_sync.llvm.16514401384283632983+0xe8/0x144 kthread_cancel_delayed_work_sync+0x18/0x2c xxxx_pm_notify+0xb0/0xd8 blocking_notifier_call_chain_robust+0x80/0x194 pm_notifier_call_chain_robust+0x28/0x4c suspend_prepare+0x40/0x260 enter_state+0x80/0x3f4 pm_suspend+0x60/0xdc state_store+0x108/0x144 kobj_attr_store+0x38/0x88 sysfs_kf_write+0x64/0xc0 kernfs_fop_write_iter+0x108/0x1d0 vfs_write+0x2f4/0x368 ksys_write+0x7c/0xec

When we started investigating, we found race between kthread_mod_delayed_work vs kthread_cancel_delayed_work_sync. The race's result could be simply reproduced as a kthread_mod_delayed_work with a following kthread_flush_work call.

Thing is we release kthread_mod_delayed_work kspin_lock in __kthread_cancel_work so it opens a race window for kthread_cancel_delayed_work_sync to change the canceling count used to prevent dwork from being requeued before calling kthread_flush_work. However, we don't check the canceling count after returning from __kthread_cancel_work and then insert the dwork to the worker. It results the following kthread_flush_work inserts flush work to dwork's tail which is at worker's dealyed_work_list. Therefore, flush work will never get moved to the worker's work_list to be executed. Finally, kthread_cancel_delayed_work_sync will NOT be able to get completed and wait forever. The code sequence diagram is as following

Thread A Thread B kthread_mod_delayed_work spin_lock __kthread_cancel_work canceling = 1 spin_unlock kthread_cancel_delayed_work_sync spin_lock kthread_cancel_work canceling = 2 spin_unlock del_timer_sync spin_lock canceling = 1 // canceling count gets update in ThreadB before queue_delayed_work // dwork is put into the woker's dealyed_work_list without checking the canceling count spin_unlock kthread_flush_work spin_lock Insert flush work // at the tail of the dwork which is at the worker's dealyed_work_list spin_unlock wait_for_completion // Thread B stuck here as flush work will never get executed

The canceling count could change in __kthread_cancel_work as the spinlock get released and regained in between, let's check the count again before we queue the delayed work to avoid the race.

Link: https://lkml.kernel.org/r/20210513065458.941403-1-liumartin@google.com Fixes: 37be45d49dec2 ("kthread: allow to cancel kthread work") Signed-off-by: Martin Liu liumartin@google.com Tested-by: David Chao davidchao@google.com Reviewed-by: Petr Mladek pmladek@suse.com Cc: Tejun Heo tj@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Ingo Molnar mingo@redhat.com Cc: Peter Zijlstra peterz@infradead.org Cc: Steven Rostedt rostedt@goodmis.org Cc: "Paul E. McKenney" paulmck@linux.vnet.ibm.com Cc: Josh Triplett josh@joshtriplett.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Jiri Kosina jkosina@suse.cz Cc: Borislav Petkov bp@suse.de Cc: Michal Hocko mhocko@suse.cz Cc: Vlastimil Babka vbabka@suse.cz Cc: Nathan Chancellor nathan@kernel.org Cc: Nick Desaulniers ndesaulniers@google.com Cc: jenhaochen@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

kernel/kthread.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)

--- a/kernel/kthread.c~kthread-fix-kthread_mod_delayed_work-vs-kthread_cancel_delayed_work_sync-race +++ a/kernel/kthread.c @@ -1181,6 +1181,19 @@ bool kthread_mod_delayed_work(struct kth goto out;

ret = __kthread_cancel_work(work, true, &flags); + + /* + * Canceling could run in parallel from kthread_cancel_delayed_work_sync + * and change work's canceling count as the spinlock is released and regain + * in __kthread_cancel_work so we need to check the count again. Otherwise, + * we might incorrectly queue the dwork and further cause + * cancel_delayed_work_sync thread waiting for flush dwork endlessly. + */ + if (work->canceling) { + ret = false; + goto out; + } + fast_queue: __kthread_queue_delayed_work(worker, dwork, delay); out: _

Patches currently in -mm which might be from liumartin@google.com are