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Re: [PATCH v1] qmp: don't hold ctx lock while querying blockstats

From: Kevin Wolf
Subject: Re: [PATCH v1] qmp: don't hold ctx lock while querying blockstats
Date: Fri, 10 Jul 2020 11:40:26 +0200

Am 10.07.2020 um 10:54 hat Zhenyu Ye geschrieben:
> Because the QMP command runs in the main thread, and changes
> to the aio context of iothread will only be executed in the
> main thread (they will not be in parallel), so there is no
> need a lock protection while querying blockstats.
> If we hold the lock here, while the I/O pressure is high in
> vm and the I/O returns slowly, the main thread will be stuck
> until the lock is released, which will affect the vcpu operation
> and finall cause the vm to be stuck.
> Signed-off-by: Zhenyu Ye <yezhenyu2@huawei.com>

Are you saying that you think qmp_query_blockstats() runs in the
iothread? This is not true, it runs in the main thread.

This makes dropping the lock wrong, and even dropping it shouldn't make
a difference for other things that should run in the main thread because
the main thread is still busy executing the QMP command.

Am I missing something?

(Also, a change like this is most definitely *not* trivial.)


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