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SubjectRe: [ANNOUNCE] mdb-2.6.27-rc2-ia32-08-07-08.patch
Jeremy Fitzhardinge wrote:
> Stefan Richter wrote:
>> jmerkey@wolfmountaingroup.com wrote:
>>> ftp://ftp.wolfmountaingroup.org/pub/mdb/mdb-2.6.27-rc2-ia32-08-07-08.patch
[...]
>> The accessors rspin_lock() and rspin_try_lock() peek into spinlock_t and
>> may therefore not be fully portable. Also, they and rspin_unlock()
>> don't look SMP safe:
>>
>>
>>> +//
>>> +// returns 0 - atomic lock occurred, processor assigned
>>> +// 1 - recusive count increased
>>> +//
>>> +
>>> +unsigned long rspin_lock(volatile rlock_t *rlock)
>>> +{
>>> +#if defined(CONFIG_SMP)
>>> + register unsigned long proc = get_processor_id();
>>> + register unsigned long retCode;
>>> +
>>> + if (rlock->lock.raw_lock.slock && rlock->processor == proc)
>>>
>
> Ticket locks will almost always have a non-zero slock. It doesn't
> indicate anything about the locked/unlocked state. But this looks like
> it's effectively doing a trylock:
>
> if (!spin_trylock(rlock) && rlock->processor == proc) {
> rlock->count++;
> ...
> } else {
> rlock->processor = proc;
> ...
> }

Right. This implemention also looks free of race conditions, provided that

- rspin_lock, rspin_try_lock, and rspin_unlock are only called in
contexts with disabled preemption and disabled local interrupts,

- rspin_unlock() rewrites rlock->processor to "no CPU" before
it drops the lock. (The implementation in
mdb-2.6.27-rc2-ia32-08-07-08.patch does so.)

BTW, the rspin_try_lock() in that patch wrong: It always returns 0
instead of having three branches of execution which return 0/1/-1.
--
Stefan Richter
-=====-==--- =--- -=--=
http://arcgraph.de/sr/


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