From: Paolo Bonzini <pbonzini@redhat.com>
To: Emanuele Giuseppe Esposito <eesposit@redhat.com>, qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH 0/5] blkdebug: fix racing condition when iterating on
Date: Tue, 13 Apr 2021 09:50:52 +0200 [thread overview]
Message-ID: <4ca1d18d-0a0e-ff52-8935-5ca9fad179cd@redhat.com> (raw)
In-Reply-To: <20210408155913.53235-1-eesposit@redhat.com>
On 08/04/21 17:59, Emanuele Giuseppe Esposito wrote:
> When qemu_coroutine_enter is executed in a loop
> (even QEMU_FOREACH_SAFE), the new routine can modify the list,
> for example removing an element, causing problem when control
> is given back to the caller that continues iterating on the same list.
>
> Patch 1 solves the issue in blkdebug_debug_resume by restarting
> the list walk after every coroutine_enter if list has to be fully iterated.
> Patches 2,3,4 aim to fix blkdebug_debug_event by gathering
> all actions that the rules make in a counter and invoking
> the respective coroutine_yeld only after processing all requests.
> Patch 5 adds a lock to protect rules and suspended_reqs.
Patch 5 is somewhat independent of the others; right now everything
works because it's protected by the AioContext lock.
On the other hand the scenarios in patches 1-4 are bugs even without
patch 5. They become more obvious if you see an explicit unlock/lock
pair within QTAILQ_FOREACH_SAFE, but they can happen already with just a
qemu_coroutine_yield or qemu_coroutine_enter within the iteration.
Paolo
prev parent reply other threads:[~2021-04-13 7:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 15:59 [PATCH 0/5] blkdebug: fix racing condition when iterating on Emanuele Giuseppe Esposito
2021-04-08 15:59 ` [PATCH 1/5] blkdebug: refactor removal of a suspended request Emanuele Giuseppe Esposito
2021-04-08 15:59 ` [PATCH 2/5] blkdebug: move post-resume handling to resume_req_by_tag Emanuele Giuseppe Esposito
2021-04-08 15:59 ` [PATCH 3/5] blkdebug: track all actions Emanuele Giuseppe Esposito
2021-04-08 15:59 ` [PATCH 4/5] blkdebug: do not suspend in the middle of QLIST_FOREACH_SAFE Emanuele Giuseppe Esposito
2021-04-13 7:48 ` Paolo Bonzini
2021-04-08 15:59 ` [PATCH 5/5] blkdebug: protect rules and suspended_reqs with a lock Emanuele Giuseppe Esposito
2021-04-13 7:50 ` Paolo Bonzini [this message]
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