On Sun, 10 Aug 2014 22:55:44 +0800, Liu Bo wrote: >>>>> This part of the trace is relatively new because Liu Bo's patch made us >>>>> redirty the pages, making it more likely that we'd try to write them >>>>> during commit. >>>>> >>>>> But, at the end of the day we have a fundamental deadlock with >>>>> committing a transaction while holding a locked page from an ordered file. >>>>> >>>>> For now, I'm ripping out the strict ordered file and going back to a >>>>> best-effort filemap_flush like ext4 is using. >>>> >>>> I think I've figured the deadlock out, this is obviously a race case, really >>>> hard to reproduce :-( >>>> >>>> So it turns out to be related to workqueues -- now a kthread can process >>>> work_struct queued in different workqueues, so we can explain the deadlock as >>>> such, >>>> >>>> (1) >>>> "btrfs-delalloc" workqueue gets a compressed extent to process with all its pages >>>> locked during this, and it runs into read free space cache inode, and then wait >>>> on lock_page(). >>>> >>>> (2) >>>> Reading that free space cache inode comes to submit part, and we have a >>>> indirect twice endio way for it, with the first endio we come to end_workqueue_bio() >>>> and queue a work in "btrfs-endio-meta" workqueue, and it will run the real >>>> endio() for us, but ONLY when it's processed. >>>> >>>> So the problem is a kthread can serve several workqueues, which means >>>> works in "btrfs-endio-meta" workqueues and works in "btrfs-flush_delalloc" >>>> workqueues can be in the same processing list of a kthread. When >>>> "btrfs-flush_delalloc" waits for the compressed page and "btrfs-endio-meta" >>>> comes after it, it hangs. >>> >>> I don't think it is right. All the btrfs workqueue has RECLAIM flag, which means >>> each btrfs workqueue has its own rescue worker. So the problem you said should >>> not happen. >> >> Right, I traded some emails with Tejun about this and spent a few days >> trying to prove the workqueues were doing the wrong thing. It will end >> up spawning another worker thread for the new work, and it won't get >> queued up behind the existing thread. >> >> If both work items went to the same workqueue, you'd definitely be right. >> >> I've got a patch to change the flush-delalloc code so we don't do the >> file writes during commit. It seems like the only choice right now. > > Not the only choice any more ;) > > It turns out to be related to async_cow's ordered list, say we have two > async_cow works on the wq->ordered_list, and the first work(named A) finishes its > ->ordered_func() and ->ordered_free(), and the second work(B) starts B's > ->ordered_func() which gets to read free space cache inode, where it queues a > work on @endio_meta_workers, but this work happens to be the same address with > A's work. > > So now the situation is, > > (1) in kthread's looping worker_thread(), work A is actually running its job, > (2) however, work A has freed its memory but kthread still want to use this > address of memory, which means worker->current_work is still A's address. > (3) B's readahead for free space cache inode happens to queue a work whose > address of memory is just the previous address of A's work, which means > another worker's ->current_work is also A's address. > (4) as in btrfs we all use function normal_work_helper(), so > worker->current_func is fixed here. > (5) worker_thread() > ->process_one_work() > ->find_worker_executing_work() > (find a collision, another work returns) > > Then we saw the hang. Here is my understand of what you said: The same worker dealt with work A and work B, and the 3rd work which was introduced by work B and has the same virtual memory address as work A was also inserted into the work list of that worker. But work B was wait for the 3rd work at that time, so deadlock happened. Am I right? If I'm right, I think what you said is impossible. Before we dealt with work B, we should already invoke spin_unlock_irq(&pool->lock), which implies a memory barrier that all changes happens before unlock should complete before unlock, that is the address in current_work should be the address of work B, when we inserted the 3rd work which was introduced by work B, we should not find the address of work A in current_work of work B's worker. I can not reproduce the problem on my machine, so I don't verify whether what I said is right or not. Please correct me if I am wrong. Thanks Miao -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
