linux-kernel - Re: [ANNOUNCE] mdb-2.6.27-rc2-ia32-08-07-08.patch

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Date:	Sat, 09 Aug 2008 10:04:03 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	jmerkey@...fmountaingroup.com, linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] mdb-2.6.27-rc2-ia32-08-07-08.patch

Jeremy Fitzhardinge wrote:
> Stefan Richter wrote:
>> jmerkey@...fmountaingroup.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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