Re: [PATCH rcu 02/18] rcu: Fix stall-warning deadlock due to non-release of rcu_node ->lock

[Paul E. McKenney]

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@xxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> 
> 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@xxxxxxx>

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!  ;-)

							Thanx, Paul