On 4/2/20 7:10 PM, Kevin Wolf wrote: > Am 02.04.2020 um 18:47 hat Kevin Wolf geschrieben: >> So I think this is the bug: Calling blk_wait_while_drained() from >> anywhere between blk_inc_in_flight() and blk_dec_in_flight() is wrong >> because it will deadlock the drain operation. >> >> blk_aio_read/write_entry() take care of this and drop their reference >> around blk_wait_while_drained(). But if we hit the race condition that >> drain hasn't yet started there, but it has when we get to >> blk_co_preadv() or blk_co_pwritev_part(), then we're in a buggy code >> path. > > With the following patch, it seems to survive for now. I'll give it some > more testing tomorrow (also qemu-iotests to check that I didn't > accidentally break something else.) >
So I only followed the discussion loosely, but tried some simple reproducing to ensure it was an issue independent of some artifacts on Dietmar's setup. Before that patch I got always a hang before reaching the fifth drive-backup + block-job-cancel cycle. With your patch applied I had no hang so far, currently into >885 cycles (and yes I confirmed stress -d 5 was really running). So, FWIW, the patch definitively fixes the issue or at least the symptoms here, I cannot comment on its correctness or the like at all, as I'm currently missing to much background. cheers, Thomas
