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locking/rwsem: Fix (possible) missed wakeup 

Because wake_q_add() can imply an immediate wakeup (cmpxchg failure
case), we must not rely on the wakeup being delayed. However, commit:

  e38513905e ("locking/rwsem: Rework zeroing reader waiter->task")

relies on exactly that behaviour in that the wakeup must not happen
until after we clear waiter->task.

[ peterz: Added changelog. ]

Signed-off-by: Xie Yongji <xieyongji@baidu.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: e38513905e ("locking/rwsem: Rework zeroing reader waiter->task")
Link: https://lkml.kernel.org/r/1543495830-2644-1-git-send-email-xieyongji@baidu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Xie Yongji 2018-11-29 20:50:30 +08:00 committed by Ingo Molnar
parent b061c38bef
commit e158488be2
1 changed files with 9 additions and 2 deletions
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11
kernel/locking/rwsem-xadd.c
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@ -198,15 +198,22 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
woken++;
tsk = waiter->task;
wake_q_add(wake_q, tsk);
get_task_struct(tsk);
list_del(&waiter->list);
/*
* Ensure that the last operation is setting the reader
* Ensure calling get_task_struct() before setting the reader
* waiter to nil such that rwsem_down_read_failed() cannot
* race with do_exit() by always holding a reference count
* to the task to wakeup.
*/
smp_store_release(&waiter->task, NULL);
/*
* Ensure issuing the wakeup (either by us or someone else)
* after setting the reader waiter to nil.
*/
wake_q_add(wake_q, tsk);
/* wake_q_add() already take the task ref */
put_task_struct(tsk);
}
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;