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    Subject[PATCHv3 03/14] writeback: do not lose wake-ups in the forker thread - 1
    Date
    From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

    Currently the forker thread can lose wake-ups which may lead to unnecessary
    delays in processing bdi works. E.g., consider the following scenario.

    1. 'bdi_forker_thread()' walks the 'bdi_list', finds out there is nothing to
    do, and is about to finish the loop.
    2. A bdi thread decides to exit because it was inactive for long time.
    3. 'bdi_queue_work()' adds a work to the bdi which just exited, so it wakes up
    the forker thread.
    4. but 'bdi_forker_thread()' executes 'set_current_state(TASK_INTERRUPTIBLE)'
    and goes sleep. We lose a wake-up.

    Losing the wake-up is not fatal, but this means that the bdi work processing
    will be delayed by up to 5 sec. This race is theoretical, I never hit it, but
    it is worth fixing.

    The fix is to execute 'set_current_state(TASK_INTERRUPTIBLE)' _before_ walking
    'bdi_list', not after.

    Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    ---
    mm/backing-dev.c | 3 +--
    1 files changed, 1 insertions(+), 2 deletions(-)

    diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    index 327e36d..b1dc2d4 100644
    --- a/mm/backing-dev.c
    +++ b/mm/backing-dev.c
    @@ -342,6 +342,7 @@ static int bdi_forker_thread(void *ptr)
    wb_do_writeback(me, 0);

    spin_lock_bh(&bdi_lock);
    + set_current_state(TASK_INTERRUPTIBLE);

    /*
    * Check if any existing bdi's have dirty data without
    @@ -357,8 +358,6 @@ static int bdi_forker_thread(void *ptr)
    bdi_add_default_flusher_thread(bdi);
    }

    - set_current_state(TASK_INTERRUPTIBLE);
    -
    if (list_empty(&bdi_pending_list)) {
    unsigned long wait;

    --
    1.7.1.1


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