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Date:   Sat, 12 Mar 2022 13:15:33 +0100
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,
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        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
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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 ?