Re: [Xen-devel] [PATCH v1 6/9] spinlock: Introduce spin_lock_cb()

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/29/2017 06:28 AM, Wei Liu wrote:
> On Fri, Mar 24, 2017 at 01:05:01PM -0400, Boris Ostrovsky wrote:
>> While waiting for a lock we may want to periodically run some
>> code. We could use spin_trylock() but since it doesn't take lock
>> ticket it may take a long time until the lock is taken.
>>
>> Add spin_lock_cb() that allows us to execute a callback while waiting.
>> Also add spin_lock_kick() that will wake up the waiters.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>> ---
>>  xen/common/spinlock.c      |   20 ++++++++++++++++++++
>>  xen/include/xen/spinlock.h |    3 +++
>>  2 files changed, 23 insertions(+), 0 deletions(-)
>>
>> diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
>> index 2a06406..d1de3ca 100644
>> --- a/xen/common/spinlock.c
>> +++ b/xen/common/spinlock.c
>> @@ -129,6 +129,26 @@ static always_inline u16 
>> observe_head(spinlock_tickets_t *t)
>>      return read_atomic(&t->head);
>>  }
>>  
>> +void _spin_lock_cb(spinlock_t *lock, void (*cb)(void *), void *data)
>> +{
>> +    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;
>> +        if ( cb )
>> +            cb(data);
>> +        arch_lock_relax();
>> +    }
>> +    LOCK_PROFILE_GOT;
>> +    preempt_disable();
>> +    arch_lock_acquire_barrier();
>> +}
>> +
>>  void _spin_lock(spinlock_t *lock)
> You should be able to use _spin_lock_cb to implement _spin_lock, right?


I did consider this but decided not to do it because we'd be adding a
few extra instructions and a call on potentially hot path.

(And doing it as a #define of a spin_lock() would make things even worse).

-boris


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