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From: Artem Bityutskiy on 16 Jul 2010 09:00
From: Artem Bityutskiy <Artem.Bityutskiy(a)nokia.com>

Currently the forker thread can lose wakeups 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 'bdi_forker_thread()'.
4. 'bdi_forker_thread()' executes
'set_current_state(TASK_INTERRUPTIBLE)' and goes sleep. We
loose a wake-up.

This situation is theoretica, I never hit it, but it is worth
fixing. The fix is to execute
'set_current_state(TASK_INTERRUPTIBLE)' before walking 'bdi_list'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy(a)nokia.com>
---
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 03a3d82..14fe1bb 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -343,6 +343,7 @@ 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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Prev: [RFC][PATCH 04/16] writeback: fix possible race when shutting down bdi
Next: [RFC][PATCH 06/16] writeback: improve bdi_has_dirty_io