From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
Chengming Zhou <zhouchengming@bytedance.com>,
mingo@redhat.com, vincent.guittot@linaro.org, bristot@redhat.com,
zhaolei@cn.fujitsu.com, tj@kernel.org, lizefan.x@bytedance.com,
hannes@cmpxchg.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock
Date: Sat, 12 Mar 2022 13:15:33 +0100 [thread overview]
Message-ID: <20220312121533.GD6235@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20220310150152.GL4285@paulmck-ThinkPad-P17-Gen-1>
On Thu, Mar 10, 2022 at 07:01:52AM -0800, Paul E. McKenney wrote:
> > > > > ./include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
> > > >
> > > > Arguably, with the flavours folded again, rcu_dereference_check() ought
> > > > to default include rcu_read_lock_sched_held() or its equivalent I
> > > > suppose.
> > > >
> > > > Paul?
> > >
> > > That would reduce the number of warnings, but it also would hide bugs.
> > >
> > > So, are you sure you really want this?
> >
> > I don't understand... Since the flavours got merged regular RCU has it's
> > quescent state held off by preempt_disable. So how can relying on that
> > cause bugs?
>
> Somene forgets an rcu_read_lock() and there happens to be something
> like a preempt_disable() that by coincidence covers that particular
> rcu_dereference(). The kernel therefore doesn't complain. That someone
> goes on to other things, maybe even posthumously. Then some time later
> the preempt_disable() goes away, for good and sufficient reasons.
>
> Good luck figuring out where to put the needed rcu_read_lock() and
> rcu_read_unlock().
Well, that's software engineering for you. Also in that case the warning
will work as expected. Then figuring out how to fix it is not the
problem of the warning -- that worked as advertised.
(also, I don't think it'll be too hard, you just gotta figure out which
object is rcu protected -- the warning gives you this, where the lookup
happens -- again the warning helps, and how long it's used for, all
relatively well definted things)
I don't see a problem. No bugs hidden.
> > And if we can rely on that, then surely rcu_dereferenced_check() ought
> > to play by the same rules, otherwise we get silly warnings like these at
> > hand.
> >
> > Specifically, we removed the rcu_read_lock() here because this has
> > rq->lock held, which is a raw_spinlock_t which very much implies preempt
> > disable, on top of that, it's also an IRQ-safe lock and thus IRQs will
> > be disabled.
> >
> > There is no possible way for RCU to make progress.
>
> Then let's have that particular rcu_dereference_check() explicitly state
> what it needs, which seems to be either rcu_read_lock() on the one hand.
> Right now, that could be just this:
>
> p = rcu_dereference_check(gp, rcu_read_lock_sched_held());
>
> Or am I missing something here?
That will work; I just don't agree with it. Per the rules of RCU it is
entirely correct to mix rcu_read_lock() and preempt_disable() (or
anything that implies the same). So I strongly feel that
rcu_dereference() should not warn about obviously correct code. Why
would we need to special case this ?
next prev parent reply other threads:[~2022-03-12 12:16 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-20 5:14 [PATCH v3 1/3] sched/cpuacct: fix charge percpu cpuusage Chengming Zhou
2022-02-20 5:14 ` [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock Chengming Zhou
2022-03-01 15:24 ` [tip: sched/core] sched/cpuacct: Optimize " tip-bot2 for Chengming Zhou
[not found] ` <CGME20220308232034eucas1p2b0f39cee0f462af6004ebdfbe5bacb9f@eucas1p2.samsung.com>
2022-03-08 23:20 ` [PATCH v3 2/3] sched/cpuacct: optimize " Marek Szyprowski
2022-03-08 23:32 ` Peter Zijlstra
2022-03-08 23:44 ` Paul E. McKenney
2022-03-09 0:21 ` Paul E. McKenney
2022-03-10 8:45 ` Peter Zijlstra
2022-03-10 15:01 ` Paul E. McKenney
2022-03-12 12:15 ` Peter Zijlstra [this message]
2022-03-12 17:44 ` Paul E. McKenney
2022-03-09 3:08 ` [External] " Chengming Zhou
2022-02-20 5:14 ` [PATCH v3 3/3] sched/cpuacct: remove redundant " Chengming Zhou
2022-03-01 15:24 ` [tip: sched/core] sched/cpuacct: Remove " tip-bot2 for Chengming Zhou
[not found] ` <CGME20220308233107eucas1p119a2f5a8d4f5b5eec38ea8dde92b3368@eucas1p1.samsung.com>
2022-03-08 23:31 ` [PATCH v3 3/3] sched/cpuacct: remove " Marek Szyprowski
2022-02-22 18:01 ` [PATCH v3 1/3] sched/cpuacct: fix charge percpu cpuusage Tejun Heo
2022-02-23 9:11 ` Peter Zijlstra
2022-03-01 15:24 ` [tip: sched/core] sched/cpuacct: Fix " tip-bot2 for Chengming Zhou
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