Commit b4df4582 authored Nov 29, 2018 by Xie Yongji Committed by Greg Kroah-Hartman Mar 05, 2019

locking/rwsem: Fix (possible) missed wakeup



[ Upstream commit e158488be27b157802753a59b336142dc0eb0380 ]

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

  e3851390 ("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: e3851390 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent d0038e4d

kernel/locking/rwsem-xadd.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -195,15 +195,22 @@ 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;