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SubjectRe: frlock and barrier discussion
On Wed, Jan 29, 2003 at 05:15:55PM -0800, Stephen Hemminger wrote:
> On Tue, 2003-01-28 at 23:06, Richard Henderson wrote:
> > On Tue, Jan 28, 2003 at 03:42:21PM -0800, Stephen Hemminger wrote:
> > > +static inline void fr_write_begin(frlock_t *rw)
> > > +{
> > > + preempt_disable();
> > > + rw->pre_sequence++;
> > > + wmb();
> > > +}
> > > +
> > > +static inline void fr_write_end(frlock_t *rw)
> > > +{
> > > + wmb();
> > > + rw->post_sequence++;
> >
> > These need to be mb(), not wmb(), if you want the bits in between
> > to actually happen in between, as with your xtime example. At
> > present there's nothing stoping xtime from being *read* before
> > your read from pre_sequence happens.
>
>
> First, write_begin/end can only be safely used when there is separate
> writer synchronization such as a spin_lock or semaphore.
> As far as I know, semaphore or spin_lock guarantees a barrier.
> So xtime or anything else can not be read before the spin_lock.
>
> Using mb() is more paranoid than necessary.

yes, it should only generate a superflous lock on x86.

it shouldn't even be necessary in fr_write_trylock.

Andrea
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