Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 08, 2022 at 03:44:03PM -0800, Paul E. McKenney wrote:
> On Wed, Mar 09, 2022 at 12:32:25AM +0100, Peter Zijlstra wrote:
> > On Wed, Mar 09, 2022 at 12:20:33AM +0100, Marek Szyprowski wrote:
> > > On 20.02.2022 06:14, Chengming Zhou wrote:
> > > > Since cpuacct_charge() is called from the scheduler update_curr(),
> > > > we must already have rq lock held, then the RCU read lock can
> > > > be optimized away.
> > > >
> > > > And do the same thing in it's wrapper cgroup_account_cputime(),
> > > > but we can't use lockdep_assert_rq_held() there, which defined
> > > > in kernel/sched/sched.h.
> > > >
> > > > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > > Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> > > 
> > > This patch landed recently in linux-next as commit dc6e0818bc9a 
> > > ("sched/cpuacct: Optimize away RCU read lock"). On my test systems I 
> > > found that it triggers a following warning in the early boot stage:
> > > 
> > > Calibrating delay loop (skipped), value calculated using timer 
> > > frequency.. 48.00 BogoMIPS (lpj=240000)
> > > pid_max: default: 32768 minimum: 301
> > > Mount-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
> > > Mountpoint-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
> > > CPU: Testing write buffer coherency: ok
> > > CPU0: Spectre v2: using BPIALL workaround
> > > 
> > > =============================
> > > WARNING: suspicious RCU usage
> > > 5.17.0-rc5-00050-gdc6e0818bc9a #11458 Not tainted
> > > -----------------------------
> > > ./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?

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.