io-wq: fix wakeup race when adding new work

When new work is added, io_wqe_enqueue() checks if we need to wake or
create a new worker. But that check is done outside the lock that
otherwise synchronizes us with a worker going to sleep, so we can end
up in the following situation:

CPU0				CPU1
lock
insert work
unlock
atomic_read(nr_running) != 0
				lock
				atomic_dec(nr_running)
no wakeup needed

Hold the wqe lock around the "need to wakeup" check. Then we can also get
rid of the temporary work_flags variable, as we know the work will remain
valid as long as we hold the lock.

Cc: stable@vger.kernel.org
Reported-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 13aeb48..cd9bd09 100644 (file)
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -794,7 +794,7 @@ append:
 static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
 {
        struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
-       int work_flags;
+       bool do_wake;
 
        /*
         * If io-wq is exiting for this task, or if the request has explicitly
@@ -806,14 +806,14 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
                return;
        }
 
-       work_flags = work->flags;
        raw_spin_lock(&wqe->lock);
        io_wqe_insert_work(wqe, work);
        wqe->flags &= ~IO_WQE_FLAG_STALLED;
+       do_wake = (work->flags & IO_WQ_WORK_CONCURRENT) ||
+                       !atomic_read(&acct->nr_running);
        raw_spin_unlock(&wqe->lock);
 
-       if ((work_flags & IO_WQ_WORK_CONCURRENT) ||
-           !atomic_read(&acct->nr_running))
+       if (do_wake)
                io_wqe_wake_worker(wqe, acct);
 }