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Date:   Fri, 6 Aug 2021 10:57:30 +0100
From:   Qais Yousef <qais.yousef@arm.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        Yanfei Xu <yanfei.xu@windriver.com>
Subject: Re: [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to
 non-release of rcu_node ->lock
Message-ID: <20210806095730.tw3bgnjtsytrqqfq@e107158-lin.cambridge.arm.com>
References: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1>
 <20210721202127.2129660-2-paulmck@kernel.org>
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On 07/21/21 13:21, Paul E. McKenney wrote:
> From: Yanfei Xu <yanfei.xu@windriver.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@windriver.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.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@arm.com>

They have Fixes tags, so should end up in 5.10 stable I presume.

Thanks!

--
Qais Yousef