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:

  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 <[email protected]>
Signed-off-by: Zhang Yu <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Fixes: e38513905eea ("locking/rwsem: Rework zeroing reader waiter->task")
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 09b180063ee11681f30a0f7e9b71a01acb3e7cc0..50d9af615dc49850acb3e0dd263cebf14130a8b6 100644 (file)
--- 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;