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(); moreover, the memory-barrier annotation "B" is used also for the smp_mb() preceding the __this_cpu_dec() in __percpu_up_read(). 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. >
