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Message-ID: <20210925125025.GO880162@paulmck-ThinkPad-P17-Gen-1> Date: Sat, 25 Sep 2021 05:50:25 -0700 From: "Paul E. McKenney" <paulmck@...nel.org> To: Boqun Feng <boqun.feng@...il.com> Cc: Waiman Long <llong@...hat.com>, peterz@...radead.org, mingo@...hat.com, will@...nel.org, linux-kernel@...r.kernel.org, richard@....at Subject: Re: Confusing lockdep splat On Sat, Sep 25, 2021 at 10:38:28AM +0800, Boqun Feng wrote: > On Fri, Sep 24, 2021 at 03:43:37PM -0700, Paul E. McKenney wrote: > > On Fri, Sep 24, 2021 at 05:41:17PM -0400, Waiman Long wrote: > > > On 9/24/21 5:02 PM, Paul E. McKenney wrote: > > > > Hello! > > > > > > > > I got the lockdep splat below from an SRCU-T rcutorture run, which uses > > > > a !SMP !PREEMPT kernel. This is a random event, and about half the time > > > > it happens within an hour or two. My reproducer (on current -rcu "dev" > > > > branch for a 16-CPU system) is: > > > > > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 16 --configs "16*SRCU-T" --duration 7200 > > > > > > > > My points of confusion are as follows: > > > > > > > > 1. The locks involved in this deadlock cycle are irq-disabled > > > > raw spinlocks. The claimed deadlock cycle uses two CPUs. > > > > There is only one CPU. There is no possibility of preemption > > > > or interrupts. So how can this deadlock actually happen? > > > > > > > > 2. If there was more than one CPU, then yes, there would be > > > > a deadlock. The PI lock is acquired by the wakeup code after > > > > acquiring the workqueue lock, and rcutorture tests the new ability > > > > of the scheduler to hold the PI lock across rcu_read_unlock(), > > > > and while it is at it, across the rest of the unlock primitives. > > > > > > > > But if there was more than one CPU, Tree SRCU would be used > > > > instead of Tiny SRCU, and there would be no wakeup invoked from > > > > srcu_read_unlock(). > > > > > > > > Given only one CPU, there is no way to complete the deadlock > > > > cycle. > > > > > > > > For now, I am working around this by preventing rcutorture from holding > > > > the PI lock across Tiny srcu_read_unlock(). > > > > > > > > Am I missing something subtle here? > > > > > > I would say that the lockdep code just doesn't have enough intelligence to > > > identify that deadlock is not possible in this special case. There are > > > certainly false positives, and it can be hard to get rid of them. > > > > Would it make sense for lockdep to filter out reports involving more > > than one CPU unless there is at least one sleeplock in the cycle? > > I think SRCU is special here, because it has different implementations > in SMP and UP. For other code, if the implemenation in SMP and UP is the > same, we want lockdep to detect the deadlock even if it's not in UP. Ah, fair point! There are a few others, for example, kernel/up.c, but it seems to just disable interrupts as its "big UP kernel lock". > We can provide an annotation similar to data_race() for SRCU to mark > UP-only code > > #define LOCKDEP_UP_ONLY(expr) ({ \ > BUILD_BUG_ON(IS_ENABLED(CONFIG_SMP)); \ > > <disable lockdep> > <...> v = expr; > <enable lockdep> > v > }) > > and in __srcu_read_unlock(): > > LOCKDEP_UP_ONLY(swake_up_one(...)); > > Thoughts? With the workaround I have now, all is well unless someone needs to hold a PI lock across an rcu_read_unlock(), which seems unlikely. If such a case does arise, lockdep will let us know. In which case what you are suggesting might be a good way to go. Alternatively, I could use the trick that RCU Tasks Trace uses, with the swake_up_one() deferred to an irq_work_queue() handler. It does appear that !SMP kernels are nowhere near as important to the community as they were 20 years ago. ;-) Thanx, Paul
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