linux-kernel - Re: [PATCH] kvm: x86: keep srcu writer side operation mutually exclusive

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Message-ID: <Y0BeEVxkDkctmTIX@google.com>
Date:   Fri, 7 Oct 2022 17:12:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Hao Peng <flyingpenghao@...il.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: keep srcu writer side operation mutually
 exclusive

On Sat, Oct 08, 2022, Hao Peng wrote:
> From: Peng Hao <flyingpeng@...cent.com>
> 
> Synchronization operations on the writer side of SRCU should be
> invoked within the mutex.

Why?  Synchronizing SRCU is necessary only to ensure that all previous readers go
away before the old filter is freed.  There's no need to serialize synchronization
between writers.  The mutex ensures each writer operates on the "new" filter that's
set by the previous writer, i.e. there's no danger of a double-free.  And the next
writer will wait for readers to _its_ "new" filter.

I think it's a moot point though, as this is a subset of patch I posted[*] to fix
other issues with the PMU event filter.

[*] https://lore.kernel.org/all/20220923001355.3741194-2-seanjc@google.com