Re: [Xen-devel] spinlock wakeup

[Julien Grall <julien.grall@xxxxxxx>]

Hi Boris,

On 24/02/17 16:15, Boris Ostrovsky wrote:
> Looking at
>
> void _spin_lock(spinlock_t *lock)
> {
>     spinlock_tickets_t tickets = SPINLOCK_TICKET_INC;
>     LOCK_PROFILE_VAR;
>
>     check_lock(&lock->debug);
>     tickets.head_tail = arch_fetch_and_add(&lock->tickets.head_tail,
>                                            tickets.head_tail);
>     while ( tickets.tail != observe_head(&lock->tickets) )
>     {
>         LOCK_PROFILE_BLOCK;
>         arch_lock_relax();
>     }
>     LOCK_PROFILE_GOT;
>     preempt_disable();
>     arch_lock_acquire_barrier();
> }
>
> If the lock is dropped during LOCK_PROFILE_BLOCK, isn't there a chance
> that we are never woken up from arch_lock_relax()? (This would only be
> an issue on ARM which IIUIC sleeps there).
The function arch_lock_signal will broadcast the event to all the 
processors and each of them will record the event. This event will be 
only cleared by a WFE (the implementation of arch_lock_acquire_barrier).
This means that if the event was happening during LOCK_PROFILE_BLOCK, 
the processor will not go to sleep and try again.
Cheers,

--
Julien Grall

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