writeback: fix possible race when creating bdi threads
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>
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@ -331,8 +331,8 @@ static int bdi_forker_thread(void *ptr)
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set_user_nice(current, 0);
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for (;;) {
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struct task_struct *task;
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struct backing_dev_info *bdi, *tmp;
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struct bdi_writeback *wb;
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/*
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* Temporary measure, we want to make sure we don't see
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@ -383,29 +383,23 @@ static int bdi_forker_thread(void *ptr)
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list_del_init(&bdi->bdi_list);
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spin_unlock_bh(&bdi_lock);
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wb = &bdi->wb;
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wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s",
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dev_name(bdi->dev));
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/*
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* If thread creation fails, then readd the bdi to
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* the pending list and force writeout of the bdi
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* from this forker thread. That will free some memory
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* and we can try again.
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*/
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if (IS_ERR(wb->task)) {
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wb->task = NULL;
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task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
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dev_name(bdi->dev));
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if (IS_ERR(task)) {
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/*
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* Add this 'bdi' to the back, so we get
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* a chance to flush other bdi's to free
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* memory.
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* If thread creation fails, then readd the bdi back to
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* the list and force writeout of the bdi from this
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* forker thread. That will free some memory and we can
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* try again. Add it to the tail so we get a chance to
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* flush other bdi's to free memory.
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*/
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spin_lock_bh(&bdi_lock);
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list_add_tail(&bdi->bdi_list, &bdi_pending_list);
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spin_unlock_bh(&bdi_lock);
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bdi_flush_io(bdi);
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}
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} else
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bdi->wb.task = task;
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}
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return 0;
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