Re: [PATCH 1/2] rwsem: check the lock before cpmxchg indown_write_trylock and rwsem_do_wake

[Tim Chen]

On Sat, 2013-06-22 at 08:10 +0800, Alex Shi wrote:
> On 06/22/2013 07:51 AM, Tim Chen wrote:
> > Doing cmpxchg will cause cache bouncing when checking
> > sem->count. This could cause scalability issue
> > in a large machine (e.g. a 80 cores box).
> > 
> > A pre-read of sem->count can mitigate this.
> > 
> > Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 
> Hi Tim,
> there is a technical error in this patch.
> the "From: " line should be 'Alex Shi', since he made the most input of
> this patch.
> 

Actually in my local git repository, I have the From line as from you
that's reflected in the change log of patch[0/2].
I'll put this as your signed-off only if you preferred for v2.  The
"From" field was you in my git but changes when I send them out.


> And I still think split this patch to 4 smaller will make it more simple
> to review, that I had sent you and Davidlohr.
> 
> could you like to re-send with my 4 patch version? :)
> 
> > ---
> >  include/asm-generic/rwsem.h |    8 ++++----
> >  lib/rwsem.c                 |   21 +++++++++++++--------
> >  2 files changed, 17 insertions(+), 12 deletions(-)
> > 
> > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> > index bb1e2cd..052d973 100644
> > --- a/include/asm-generic/rwsem.h
> > +++ b/include/asm-generic/rwsem.h
> > @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)
> >  
> >  static inline int __down_write_trylock(struct rw_semaphore *sem)
> >  {
> > -	long tmp;
> > +	if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
> > +		return 0;
> >  
> > -	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> > -		      RWSEM_ACTIVE_WRITE_BIAS);
> > -	return tmp == RWSEM_UNLOCKED_VALUE;
> > +	return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> > +		RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
> >  }
> >  
> >  /*
> > diff --git a/lib/rwsem.c b/lib/rwsem.c
> > index 19c5fa9..2072af5 100644
> > --- a/lib/rwsem.c
> > +++ b/lib/rwsem.c
> > @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
> >  			 * will block as they will notice the queued writer.
> >  			 */
> >  			wake_up_process(waiter->task);
> > -		goto out;
> > +		return sem;
> >  	}
> >  
> >  	/* Writers might steal the lock before we grant it to the next reader.
> > @@ -85,15 +85,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
> >  	adjustment = 0;
> >  	if (wake_type != RWSEM_WAKE_READ_OWNED) {
> >  		adjustment = RWSEM_ACTIVE_READ_BIAS;
> > - try_reader_grant:
> > -		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
> > -		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
> > -			/* A writer stole the lock. Undo our reader grant. */
> > +		while (1) {
> > +			/* A writer stole the lock. */
> > +			if (sem->count < RWSEM_WAITING_BIAS)
> > +				return sem;
> > +
> > +			oldcount = rwsem_atomic_update(adjustment, sem)
> > +								- adjustment;
> > +			if (likely(oldcount >= RWSEM_WAITING_BIAS))
> > +				break;
> > +
> > +			 /* A writer stole the lock.  Undo our reader grant. */
> >  			if (rwsem_atomic_update(-adjustment, sem) &
> >  						RWSEM_ACTIVE_MASK)
> > -				goto out;
> > +				return sem;
> >  			/* Last active locker left. Retry waking readers. */
> > -			goto try_reader_grant;
> >  		}
> >  	}
> >  
> > @@ -136,7 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
> >  	sem->wait_list.next = next;
> >  	next->prev = &sem->wait_list;
> >  
> > - out:
> >  	return sem;
> >  }
> >  
> > 
> 
> 


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