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    Subject[PATCH 5.12 209/242] rq-qos: fix missed wake-ups in rq_qos_throttle try two
    Date
    From: Jan Kara <jack@suse.cz>

    commit 11c7aa0ddea8611007768d3e6b58d45dc60a19e1 upstream.

    Commit 545fbd0775ba ("rq-qos: fix missed wake-ups in rq_qos_throttle")
    tried to fix a problem that a process could be sleeping in rq_qos_wait()
    without anyone to wake it up. However the fix is not complete and the
    following can still happen:

    CPU1 (waiter1) CPU2 (waiter2) CPU3 (waker)
    rq_qos_wait() rq_qos_wait()
    acquire_inflight_cb() -> fails
    acquire_inflight_cb() -> fails

    completes IOs, inflight
    decreased
    prepare_to_wait_exclusive()
    prepare_to_wait_exclusive()
    has_sleeper = !wq_has_single_sleeper() -> true as there are two sleepers
    has_sleeper = !wq_has_single_sleeper() -> true
    io_schedule() io_schedule()

    Deadlock as now there's nobody to wakeup the two waiters. The logic
    automatically blocking when there are already sleepers is really subtle
    and the only way to make it work reliably is that we check whether there
    are some waiters in the queue when adding ourselves there. That way, we
    are guaranteed that at least the first process to enter the wait queue
    will recheck the waiting condition before going to sleep and thus
    guarantee forward progress.

    Fixes: 545fbd0775ba ("rq-qos: fix missed wake-ups in rq_qos_throttle")
    CC: stable@vger.kernel.org
    Signed-off-by: Jan Kara <jack@suse.cz>
    Link: https://lore.kernel.org/r/20210607112613.25344-1-jack@suse.cz
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/blk-rq-qos.c | 4 ++--
    include/linux/wait.h | 2 +-
    kernel/sched/wait.c | 9 +++++++--
    3 files changed, 10 insertions(+), 5 deletions(-)

    --- a/block/blk-rq-qos.c
    +++ b/block/blk-rq-qos.c
    @@ -266,8 +266,8 @@ void rq_qos_wait(struct rq_wait *rqw, vo
    if (!has_sleeper && acquire_inflight_cb(rqw, private_data))
    return;

    - prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
    - has_sleeper = !wq_has_single_sleeper(&rqw->wait);
    + has_sleeper = !prepare_to_wait_exclusive(&rqw->wait, &data.wq,
    + TASK_UNINTERRUPTIBLE);
    do {
    /* The memory barrier in set_task_state saves us here. */
    if (data.got_token)
    --- a/include/linux/wait.h
    +++ b/include/linux/wait.h
    @@ -1136,7 +1136,7 @@ do { \
    * Waitqueues which are removed from the waitqueue_head at wakeup time
    */
    void prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state);
    -void prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state);
    +bool prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state);
    long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state);
    void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);
    long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout);
    --- a/kernel/sched/wait.c
    +++ b/kernel/sched/wait.c
    @@ -264,17 +264,22 @@ prepare_to_wait(struct wait_queue_head *
    }
    EXPORT_SYMBOL(prepare_to_wait);

    -void
    +/* Returns true if we are the first waiter in the queue, false otherwise. */
    +bool
    prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
    {
    unsigned long flags;
    + bool was_empty = false;

    wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    - if (list_empty(&wq_entry->entry))
    + if (list_empty(&wq_entry->entry)) {
    + was_empty = list_empty(&wq_head->head);
    __add_wait_queue_entry_tail(wq_head, wq_entry);
    + }
    set_current_state(state);
    spin_unlock_irqrestore(&wq_head->lock, flags);
    + return was_empty;
    }
    EXPORT_SYMBOL(prepare_to_wait_exclusive);


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