On Thu, 2019-09-12 at 18:17 -0400, Joel Fernandes wrote: > On Wed, Sep 11, 2019 at 05:57:29PM +0100, Scott Wood wrote: > > rcutorture was generating some nesting scenarios that are not > > reasonable. Constrain the state selection to avoid them. > > > > Example #1: > > > > 1. preempt_disable() > > 2. local_bh_disable() > > 3. preempt_enable() > > 4. local_bh_enable() > > > > On PREEMPT_RT, BH disabling takes a local lock only when called in > > non-atomic context. Thus, atomic context must be retained until after > > BH > > is re-enabled. Likewise, if BH is initially disabled in non-atomic > > context, it cannot be re-enabled in atomic context. > > > > Example #2: > > > > 1. rcu_read_lock() > > 2. local_irq_disable() > > 3. rcu_read_unlock() > > 4. local_irq_enable() > > If I understand correctly, these examples are not unrealistic in the real > world unless RCU is used in the scheduler. I hope you mean "not realistic", at least when it comes to explicit preempt/irq disabling rather than spinlock variants that don't disable preempt/irqs on PREEMPT_RT. > > If the thread is preempted between steps 1 and 2, > > rcu_read_unlock_special.b.blocked will be set, but it won't be > > acted on in step 3 because IRQs are disabled. Thus, reporting of the > > quiescent state will be delayed beyond the local_irq_enable(). > > Yes, with consolidated RCU this can happen but AFAIK it has not seen to be > a > problem since deferred QS reporting will happen take care of it, which can > also happen from subsequent rcu_read_unlock_special(). The defer_qs_iw_pending stuff isn't in 5.2-rt. Still, given patch 4/5 (and special.b.deferred_qs on mainline) this shouldn't present a deadlock concern (letting the test run a bit now to double check) so this patch could probably be limited to the "example #1" sequence. > > For now, these scenarios will continue to be tested on non-PREEMPT_RT > > kernels, until debug checks are added to ensure that they are not > > happening elsewhere. > > Are you seeing real issues that need this patch? It would be good to not > complicate rcutorture if not needed. rcutorture crashes on RT without this patch (in particular due to the local_bh_disable misordering). -Scott
