Re: Deadlock with ide_issue_trim and draining

[Hanna Czenczek <hreitz@redhat.com>]

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.

Hanna