Re: [PATCH v10 1/4] qrwlock: A queue read/write lock implementation

[Waiman Long]

On 01/23/2014 05:07 AM, Peter Zijlstra wrote:
> On Wed, Jan 22, 2014 at 04:33:55PM -0500, Waiman Long wrote:
> > +/**
> > + * queue_read_unlock - release read lock of a queue rwlock
> > + * @lock : Pointer to queue rwlock structure
> > + */
> > +static inline void queue_read_unlock(struct qrwlock *lock)
> > +{
> > +	/*
> > +	 * Atomically decrement the reader count
> > +	 */
> > +	atomic_sub(_QR_BIAS,&lock->cnts.rwa);
> > +}
> > +
> > +/**
> > + * queue_write_unlock - release write lock of a queue rwlock
> > + * @lock : Pointer to queue rwlock structure
> > + */
> > +static inline void queue_write_unlock(struct qrwlock *lock)
> > +{
> > +	/*
> > +	 * If the writer field is atomic, it can be cleared directly.
> > +	 * Otherwise, an atomic subtraction will be used to clear it.
> > +	 */
> > +	if (__native_word(lock->cnts.writer))
> > +		smp_store_release(&lock->cnts.writer, 0);
> > +	else
> > +		atomic_sub(_QW_LOCKED,&lock->cnts.rwa);
> > +}
> Both these unlocks miss a barrier; atomic_sub() doesn't imply any
> barrier what so ever.
>
> The smp_store_release() does, but the other two are invalid release ops
> in generic.

I thought that all atomic RMW instructions are memory barrier. If they 
are not, what kind of barrier should be added?

-Longman
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