On Fri, Dec 21, 2018 at 12:59:13PM +0530, Prateek Sood wrote: > P1 is releaseing the cpu_hotplug_lock and P2 is 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, it 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 <[email protected]> > Acked-by: Davidlohr Bueso <[email protected]>
It looks like Peter has already queued this, c.f., https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=locking/core&id=73685b8af253cf32b1b41b3045f2828c6fb2439e with a modified changelog and my Reviewed-by (that I confirm). I can't tell how/when this is going to be upstreamed (guess via -tip), Peter? Andrea > > --- > kernel/exit.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/exit.c b/kernel/exit.c > index ac1a814..696e0e1 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -298,7 +298,7 @@ void rcuwait_wake_up(struct rcuwait *w) > /* > * Order condition vs @task, such that everything prior to the load > * of @task is visible. This is the condition as to why the user called > - * rcuwait_trywake() in the first place. Pairs with set_current_state() > + * rcuwait_wake_up() in the first place. Pairs with set_current_state() > * barrier (A) in rcuwait_wait_event(). > * > * WAIT WAKE > @@ -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. >
