Deadlock with ide_issue_trim and draining

[Fiona Ebner <f.ebner@proxmox.com>]

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.

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?

Best Regards,
Fiona