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writeback: fix possible race when creating bdi threads

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This patch fixes a very unlikely race condition on the bdi forker thread error
path: when bdi thread creation fails, 'bdi->wb.task' may contain the error code
for a short period of time. If at the same time someone submits a work to this
bdi, we can end up with an oops 'bdi_queue_work()' while executing
'wake_up_process(wb->task)'.

This patch fixes the issue by introducing a temporary variable 'task' and
storing the possible error code there, so that 'wb->task' would never take
erroneous values.

Note, this race is very unlikely and I never hit it, so it is theoretical, but
nevertheless worth fixing.

This patch also merges 2 comments which were previously separate.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Artem Bityutskiy 2010-07-25 14:29:12 +03:00 committed by Jens Axboe
parent 6f904ff0e3
commit 94eac5e623
1 changed files with 11 additions and 17 deletions
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28
mm/backing-dev.c
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@ -331,8 +331,8 @@ static int bdi_forker_thread(void *ptr)
set_user_nice(current, 0);
for (;;) {
struct task_struct *task;
struct backing_dev_info *bdi, *tmp;
struct bdi_writeback *wb;
/*
* Temporary measure, we want to make sure we don't see
@ -383,29 +383,23 @@ static int bdi_forker_thread(void *ptr)
list_del_init(&bdi->bdi_list);
spin_unlock_bh(&bdi_lock);
wb = &bdi->wb;
wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s",
dev_name(bdi->dev));
/*
* If thread creation fails, then readd the bdi to
* the pending list and force writeout of the bdi
* from this forker thread. That will free some memory
* and we can try again.
*/
if (IS_ERR(wb->task)) {
wb->task = NULL;
task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
dev_name(bdi->dev));
if (IS_ERR(task)) {
/*
* Add this 'bdi' to the back, so we get
* a chance to flush other bdi's to free
* memory.
* If thread creation fails, then readd the bdi back to
* the list and force writeout of the bdi from this
* forker thread. That will free some memory and we can
* try again. Add it to the tail so we get a chance to
* flush other bdi's to free memory.
*/
spin_lock_bh(&bdi_lock);
list_add_tail(&bdi->bdi_list, &bdi_pending_list);
spin_unlock_bh(&bdi_lock);
bdi_flush_io(bdi);
}
} else
bdi->wb.task = task;
}
return 0;