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Re: [PATCH for-8.0] ide: Fix manual in-flight count for TRIM BH

From: Paolo Bonzini
Subject: Re: [PATCH for-8.0] ide: Fix manual in-flight count for TRIM BH
Date: Mon, 13 Mar 2023 14:09:17 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 3/13/23 13:29, Fiona Ebner wrote:
In fact, shouldn't request queuing was enabled at the _end_ of
bdrv_drained_begin (once the BlockBackend has reached a quiescent
state on its own terms), rather than at the beginning (which leads to
deadlocks like this one)?

Couldn't this lead to scenarios where a busy or malicious guest, which
continues to submit new requests, slows down draining or even prevents
it from finishing?

Possibly, but there is also a .drained_begin/.drained_end callback that can be defined in order to apply backpressure. (For some other devices, there's also aio_disable_external/aio_enable_external that do the equivalent of request queuing but without the deadlocks)

Since starting the queuing of requests at the end of bdrv_drained_begin wouldn't hurt correctness, and it would fix this kind of deadlock, I think it would be worth giving it a try.


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