On Tue, 2019-09-17 at 12:07 +0200, Sebastian Andrzej Siewior wrote: > On 2019-09-16 11:55:57 [-0500], Scott Wood wrote: > > 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. > > We have: > - local_irq_disable() (+save) > - spin_lock() > - local_bh_disable() > - preempt_disable() > > On non-RT you can (but should not) use the counter part of the function > in random order like: > local_bh_disable(); > local_irq_disable(); > local_bh_enable(); > local_irq_enable(); Actually even non-RT will assert if you do local_bh_enable() with IRQs disabled -- but the other combinations do work, and are used some places via spinlocks. If they are used via direct calls to preempt_disable() or local_irq_disable() (or via raw spinlocks), then that will not go away on RT and we'll have a problem. > The non-RT will survive this. On RT the counterpart functions have to be > used in reverse order: > local_bh_disable(); > local_irq_disable(); > local_irq_enable(); > local_bh_enable(); > > or the kernel will fall apart. > > Since you _can_ use it in random order Paul wants to test that the > random use of those function does not break RCU in any way. Since they > can not be used on RT in random order it has been agreed that we keep > the test for !RT but disable it on RT. For now, yes. Long term it would be good to keep track of when preemption/irqs would be disabled on RT, even when running a non-RT debug kernel, and assert when bad things are done with it (assuming an RT-capable arch). Besides detecting these fairly unusual patterns, it could also detect earlier the much more common problem of nesting a non-raw spinlock inside a raw spinlock or other RT-atomic context. -Scott
