4.20-stable review patch. If anyone has any objections, please let me know.
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[ 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:
e38513905eea ("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: e38513905eea ("locking/rwsem: Rework zeroing reader waiter->task") Link: https://lkml.kernel.org/r/1543495830-2644-1-git-send-email-xieyongji@baidu.c... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/locking/rwsem-xadd.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 09b180063ee11..50d9af615dc49 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -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;