Hello and thanks for the reply, On 10/12/2015 04:40 PM, Jan Kara wrote: > On Fri 09-10-15 11:03:30, Nikolay Borisov wrote: >> On 10/09/2015 10:37 AM, Hillf Danton wrote: >>>>>> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio) >>>>>> if (bio->bi_error) >>>>>> buffer_io_error(bh); >>>>>> } while ((bh = bh->b_this_page) != head); >>>>>> - bit_spin_unlock(BH_Uptodate_Lock, &head->b_state); >>>>>> local_irq_restore(flags); >>>>> >>>>> What if it takes 100ms to unlock after IRQ restored? >>>> >>>> I'm not sure I understand in what direction you are going? Care to >>>> elaborate? >>>> >>> Your change introduces extra time cost the lock waiter has to pay in >>> the case that irq happens before the lock is released. >> >> [CC filesystem and mm people. For reference the thread starts here: >> http://thread.gmane.org/gmane.linux.kernel/2056996 ] >> >> Right, I see what you mean and it's a good point but when doing the >> patches I was striving for correctness and starting a discussion, hence >> the RFC. In any case I'd personally choose correctness over performance >> always ;). >> >> As I'm not an fs/ext4 expert and have added the relevant parties (please >> use reply-all from now on so that the thread is not being cut in the >> middle) who will be able to say whether it impact is going to be that >> big. I guess in this particular code path worrying about this is prudent >> as writeback sounds like a heavily used path. >> >> Maybe the problem should be approached from a different angle e.g. >> drain_all_pages and its reliance on the fact that the IPI will always be >> delivered in some finite amount of time? But what if a cpu with disabled >> interrupts is waiting on the task issuing the IPI? > > So I have looked through your patch and also original report (thread starts > here: https://lkml.org/lkml/2015/10/8/341) and IMHO one question hasn't > been properly answered yet: Who is holding BH_Uptodate_Lock we are spinning > on? You have suggested in https://lkml.org/lkml/2015/10/8/464 that it was > __block_write_full_page_endio() call but that cannot really be the case. > BH_Uptodate_Lock is used only in IO completion handlers - > end_buffer_async_read, end_buffer_async_write, ext4_finish_bio. So there > really should be some end_io function running on some other CPU which holds > BH_Uptodate_Lock for that buffer. I did check all the call traces of the current processes on the machine at the time of the hard lockup and none of the 3 functions you mentioned were in any of the call chains. But while I was looking the code of end_buffer_async_write and in the comments I saw it was mentioned that those completion handler were called from __block_write_full_page_endio so that's what pointed my attention to that function. But you are right that it doesn't take the BH lock. Furthermore the fact that the BH_Async_Write flag is set points me in the direction that end_buffer_async_write should have been executing but as I said issuing "bt" for all the tasks didn't show this function. I'm beginning to wonder if it's possible that a single bit memory error has crept up, but this still seems like a long shot... Btw I think in any case the spin_lock patch is wrong as this code can be called from within softirq context and we do want to be interrupt safe at that point. > > BTW: I suppose the filesystem uses 4k blocksize, doesn't it? Unfortunately I cannot tell you with 100% certainty, since on this server there are multiple block devices with blocksize either 1k or 4k. So it is one of these. If you know a way to extract this information from a vmcore file I'd be happy to do it. > > Honza > >>>>>> + bit_spin_unlock(BH_Uptodate_Lock, &head->b_state); >>>>>> if (!under_io) { >>>>>> #ifdef CONFIG_EXT4_FS_ENCRYPTION >>>>>> if (ctx) >>>>>> -- >>>>>> 2.5.0 >>>>> >>> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>