Re: Deadlock with ide_issue_trim and draining

[Hanna Czenczek <hreitz@redhat.com>]

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?

Hanna