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

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Message-ID: <20200527012245.GJ991@lca.pw>
Date:   Tue, 26 May 2020 21:22:45 -0400
From:   Qian Cai <cai@....pw>
To:     Paul Mackerras <paulus@...abs.org>
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 Wed, May 27, 2020 at 11:13:23AM +1000, Paul Mackerras wrote:
> 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?

Feel free to take out that cond_resched_rcu(). Your reasoning makes
sense.