Re: Deadlock with ide_issue_trim and draining

[Fiona Ebner]

Am 07.03.23 um 15:27 schrieb Hanna Czenczek:
> On 07.03.23 14:44, Hanna Czenczek wrote:
>> On 07.03.23 13:22, Fiona Ebner wrote:
>>> Hi,
>>> I am suspecting that commit 7e5cdb345f ("ide: Increment BB in-flight
>>> counter for TRIM BH") introduced an issue in combination with draining.
>>>
>>>  From a debug session on a costumer's machine I gathered the following
>>> information:
>>> * The QEMU process hangs in aio_poll called during draining and doesn't
>>> progress.
>>> * The in_flight counter for the BlockDriverState is 0 and for the
>>> BlockBackend it is 1.
>>> * There is a blk_aio_pdiscard_entry request in the BlockBackend's
>>> queued_requests.
>>> * The drive is attached via ahci.
>>>
>>> I suspect that something like the following happened:
>>>
>>> 1. ide_issue_trim is called, and increments the in_flight counter.
>>> 2. ide_issue_trim_cb calls blk_aio_pdiscard.
>>> 3. somebody else starts draining.
>>> 4. ide_issue_trim_cb is called as the completion callback for
>>> blk_aio_pdiscard.
>>> 5. ide_issue_trim_cb issues yet another blk_aio_pdiscard request.
>>> 6. The request is added to the wait queue via blk_wait_while_drained,
>>> because draining has been started.
>>> 7. Nobody ever decrements the in_flight counter and draining can't
>>> finish.
>>
>> Sounds about right.
>>
>>> The issue occurs very rarely and is difficult to reproduce, but with the
>>> help of GDB, I'm able to do it rather reliably:
>>> 1. Use GDB to break on blk_aio_pdiscard.
>>> 2. Run mkfs.ext4 on a huge disk in the guest.
>>> 3. Issue a drive-backup QMP command after landing on the breakpoint.
>>> 4. Continue a few times in GDB.
>>> 5. After that I can observe the same situation as described above.
>>>
>>> I'd be happy about suggestions for how to fix it. Unfortunately, I don't
>>> see a clear-cut way at the moment. The only idea I have right now is to
>>> change the code to issue all discard requests at the same time, but I
>>> fear there might pitfalls with that?
>>
>> The point of 7e5cdb345f was that we need any in-flight count to
>> accompany a set s->bus->dma->aiocb.  While blk_aio_pdiscard() is
>> happening, we don’t necessarily need another count.  But we do need it
>> while there is no blk_aio_pdiscard().
>>
>> ide_issue_trim_cb() returns in two cases (and, recursively through its
>> callers, leaves s->bus->dma->aiocb set):
>> 1. After calling blk_aio_pdiscard(), which will keep an in-flight count,
>> 2. After calling replay_bh_schedule_event() (i.e. qemu_bh_schedule()),
>> which does not keep an in-flight count.
>>
>> Perhaps we just need to move the blk_inc_in_flight() above the
>> replay_bh_schedule_event() call?
> 
> FWIW, doing so at least keeps the reproducer from
> https://bugzilla.redhat.com/show_bug.cgi?id=2029980 working.
> 

And I'm not able to run into my current issue anymore :) Thank you!

FWIW, the suggested change and explanation sound good to me. Are you
going to send a patch for it?

Best Regards,
Fiona