linux-kernel - Re: [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to non-release of rcu

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Date:   Fri, 6 Aug 2021 13:33:46 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Yanfei Xu <yanfei.xu@...driver.com>
Subject: Re: [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to
 non-release of rcu_node ->lock

On 08/06/21 04:43, Paul E. McKenney wrote:
> On Fri, Aug 06, 2021 at 10:57:30AM +0100, Qais Yousef wrote:
> > On 07/21/21 13:21, Paul E. McKenney wrote:
> > > From: Yanfei Xu <yanfei.xu@...driver.com>
> > > 
> > > If rcu_print_task_stall() is invoked on an rcu_node structure that does
> > > not contain any tasks blocking the current grace period, it takes an
> > > early exit that fails to release that rcu_node structure's lock.  This
> > > results in a self-deadlock, which is detected by lockdep.
> > > 
> > > To reproduce this bug:
> > > 
> > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 3 --trust-make --configs "TREE03" --kconfig "CONFIG_PROVE_LOCKING=y" --bootargs "rcutorture.stall_cpu=30 rcutorture.stall_cpu_block=1 rcutorture.fwd_progress=0 rcutorture.test_boost=0"
> > > 
> > > This will also result in other complaints, including RCU's scheduler
> > > hook complaining about blocking rather than preemption and an rcutorture
> > > writer stall.
> > > 
> > > Only a partial RCU CPU stall warning message will be printed because of
> > > the self-deadlock.
> > > 
> > > This commit therefore releases the lock on the rcu_print_task_stall()
> > > function's early exit path.
> > > 
> > > Fixes: c583bcb8f5ed ("rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled")
> > > Signed-off-by: Yanfei Xu <yanfei.xu@...driver.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > ---
> > 
> > We were seeing similar issue on Android 5.10. Applying patches 1 and 2 did fix
> > the deadlock problem and we get proper RCU stall splat now.
> > 
> > For patches 1 and 2:
> > 
> > Tested-by: Qais Yousef <qais.yousef@....com>
> 
> Thank you, and I will apply this on the next rebase.
> 
> > They have Fixes tags, so should end up in 5.10 stable I presume.
> 
> Here is hoping!  ;-)

Leave it with me, I've got a todo to make sure this happens ;-)

Thanks!

--
Qais Yousef