linux-kernel - Re: [PATCH] powerpc/kvm/book3s64/vio: fix some RCU-list locks

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Message-ID: <20200527011323.GA293451@thinks.paulus.ozlabs.org>
Date:   Wed, 27 May 2020 11:13:23 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Qian Cai <cai@....pw>
Cc:     mpe@...erman.id.au, benh@...nel.crashing.org, aik@...abs.ru,
        paulmck@...nel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/kvm/book3s64/vio: fix some RCU-list locks

On Sun, May 10, 2020 at 01:18:34AM -0400, Qian Cai wrote:
> It is unsafe to traverse kvm->arch.spapr_tce_tables and
> stt->iommu_tables without the RCU read lock held. Also, add
> cond_resched_rcu() in places with the RCU read lock held that could take
> a while to finish.

This mostly looks fine.  The cond_resched_rcu() in kvmppc_tce_validate
doesn't seem necessary (the list would rarely have more than a few
dozen entries) and could be a performance problem given that TCE
validation is a hot-path.

Are you OK with me modifying the patch to take out that
cond_resched_rcu(), or is there some reason why it's essential that it
be there?

Paul.