Commit c5f7ad23 authored Jul 25, 2010 by Artem Bityutskiy Committed by Jens Axboe Aug 07, 2010

writeback: do not lose wake-ups in the forker thread - 1



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>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

parent 94eac5e6

mm/backing-dev.c

+1 −2

Original line number	Diff line number	Diff line
		@@ -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;

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