On Thu, Jun 27, 2019 at 11:30 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > On Thu, Jun 27, 2019 at 10:34:55AM -0400, Steven Rostedt wrote: > > On Thu, 27 Jun 2019 10:24:36 -0400 > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > What am I missing here? > > > > > > This issue I think is > > > > > > (in normal process context) > > > spin_lock_irqsave(rq_lock); // which disables both preemption and interrupt > > > // but this was done in normal process context, > > > // not from IRQ handler > > > rcu_read_lock(); > > > <---------- IPI comes in and sets exp_hint > > > > How would an IPI come in here with interrupts disabled? > > > > -- Steve > > This is true, could it be rcu_read_unlock_special() got called for some > *other* reason other than the IPI then? > > Per Sebastian's stack trace of the recursive lock scenario, it is happening > during cpu_acct_charge() which is called with the rq_lock held. > > The only other reasons I know off to call rcu_read_unlock_special() are if > 1. the tick indicated that the CPU has to report a QS > 2. an IPI in the middle of the reader section for expedited GPs > 3. preemption in the middle of a preemptible RCU reader section > > 1. and 2. are not possible because interrupts are disabled, that's why the > wakeup_softirq even happened. > 3. is not possible because we are holding rq_lock in the RCU reader section. > > So I am at a bit of a loss how this can happen :-( Sebastian it would be nice if possible to trace where the t->rcu_read_unlock_special is set for this scenario of calling rcu_read_unlock_special, to give a clear idea about whether it was really because of an IPI. I guess we could also add additional RCU debug fields to task_struct (just for debugging) to see where there unlock_special is set. Is there a test to reproduce this, or do I just boot an intel x86_64 machine with "threadirqs" and run into it?
