----- On Oct 5, 2017, at 6:02 PM, Andrea Parri [email protected] wrote:
> On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote: >> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra [email protected] wrote: >> >> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote: >> >> diff --git a/arch/powerpc/kernel/membarrier.c >> >> b/arch/powerpc/kernel/membarrier.c >> >> new file mode 100644 >> >> index 000000000000..b0d79a5f5981 >> >> --- /dev/null >> >> +++ b/arch/powerpc/kernel/membarrier.c >> >> @@ -0,0 +1,45 @@ >> > >> >> +void membarrier_arch_register_private_expedited(struct task_struct *p) >> >> +{ >> >> + struct task_struct *t; >> >> + >> >> + if (get_nr_threads(p) == 1) { >> >> + set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED); >> >> + return; >> >> + } >> >> + /* >> >> + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread >> >> + * fork is protected by siglock. >> >> + */ >> >> + spin_lock(&p->sighand->siglock); >> >> + for_each_thread(p, t) >> >> + set_ti_thread_flag(task_thread_info(t), >> >> + TIF_MEMBARRIER_PRIVATE_EXPEDITED); >> > >> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD. >> >> The intent here is to hold the sighand siglock to provide mutual >> exclusion against invocation of membarrier_fork(p, clone_flags) >> by copy_process(). >> >> copy_process() grabs spin_lock(¤t->sighand->siglock) for both >> CLONE_THREAD and not CLONE_THREAD flags. >> >> What am I missing here ? >> >> > >> >> + spin_unlock(&p->sighand->siglock); >> >> + /* >> >> + * Ensure all future scheduler executions will observe the new >> >> + * thread flag state for this process. >> >> + */ >> >> + synchronize_sched(); >> > >> > This relies on the flag being read inside rq->lock, right? >> >> Yes. The flag is read by membarrier_arch_switch_mm(), invoked >> within switch_mm_irqs_off(), called by context_switch() before >> finish_task_switch() releases the rq lock. > > I fail to grap the relation between this synchronize_sched() and rq->lock. > > (Besides, we made no reference to rq->lock while discussing: > > > https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b > replace membarrier_arch_sched_in with switch_mm_irqs_off ) > > Could you elaborate? Hi Andrea, AFAIU the scheduler rq->lock is held while preemption is disabled. synchronize_sched() is used here to ensure that all pre-existing preempt-off critical sections have completed. So saying that we use synchronize_sched() to synchronize with rq->lock would be stretching the truth a bit. It's actually only true because the scheduler holding the rq->lock is surrounded by a preempt-off critical section. Thanks, Mathieu > > Andrea > > >> >> Is the comment clear enough, or do you have suggestions for >> improvements ? > > > >> >> Thanks, >> >> Mathieu >> >> > >> > > +} >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. > > http://www.efficios.com -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
