On 12/12/2018 08:58 PM, Andrea Parri wrote: > On Fri, Nov 30, 2018 at 08:40:56PM +0530, Prateek Sood wrote: >> In a scenario where cpu_hotplug_lock percpu_rw_semaphore is already >> acquired for read operation by P1 using percpu_down_read(). >> >> Now we have P1 in the path of releaseing the cpu_hotplug_lock and P2 >> is in the process of acquiring cpu_hotplug_lock. >> >> P1 P2 >> percpu_up_read() path percpu_down_write() path >> >> rcu_sync_enter() >> //gp_state=GP_PASSED >> >> rcu_sync_is_idle() //returns false down_write(rw_sem) >> >> __percpu_up_read() >> >> [L] task = rcu_dereference(w->task) //NULL >> >> smp_rmb() [S] w->task = current >> >> smp_mb() >> >> [L] readers_active_check() //fails >> schedule() >> >> [S] __this_cpu_dec(read_count) >> >> Since load of task can result in NULL. This can lead to missed wakeup >> in rcuwait_wake_up(). Above sequence violated the following constraint >> in rcuwait_wake_up(): >> >> WAIT WAKE >> [S] tsk = current [S] cond = true >> MB (A) MB (B) >> [L] cond [L] tsk >> >> This can happen as smp_rmb() in rcuwait_wake_up() will provide ordering >> of load before barrier with load and store after barrier for arm64 >> architecture. Here the requirement is to order store before smp_rmb() >> with load after the smp_rmb(). >> >> For the usage of rcuwait_wake_up() in __percpu_up_read() full barrier >> (smp_mb) is required to complete the constraint of rcuwait_wake_up(). >> >> Signed-off-by: Prateek Sood <prs...@codeaurora.org> > > I know this is going to sound ridiculous (coming from me or from > the Italian that I am), but it looks like we could both work on > our English. ;-) > > But the fix seems correct to me: > > Reviewed-by: Andrea Parri <andrea.pa...@amarulasolutions.com> > > It might be a good idea to integrate this fix with fixes to the > inline comments/annotations: for example, I see that the comment > in rcuwait_wake_up() mentions a non-existing rcuwait_trywake(); Ok, I will update the comment in next version of the patch.
> moreover, the memory-barrier annotation "B" is used also for the > smp_mb() preceding the __this_cpu_dec() in __percpu_up_read(). In this annotation "B" is corresponding to annotation "A" in rcuwait_wait_event(). So this seems to be correct. > > Andrea > > >> --- >> kernel/exit.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/exit.c b/kernel/exit.c >> index f1d74f0..a10820d 100644 >> --- a/kernel/exit.c >> +++ b/kernel/exit.c >> @@ -306,7 +306,7 @@ void rcuwait_wake_up(struct rcuwait *w) >> * MB (A) MB (B) >> * [L] cond [L] tsk >> */ >> - smp_rmb(); /* (B) */ >> + smp_mb(); /* (B) */ >> >> /* >> * Avoid using task_rcu_dereference() magic as long as we are careful, >> -- >> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, >> Inc., >> is a member of Code Aurora Forum, a Linux Foundation Collaborative Project. >> -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project