Quoting r. Roland Dreier <[email protected]>:
> Subject: [openib-general] Re: [git patch review 1/4] IPoIB: Don't start send-only joins while multicast thread is stopped
>
> > Roland Dreier <[email protected]> wrote:
> > >
> > > + spin_lock_irq(&priv->lock);
> > > + set_bit(IPOIB_MCAST_STARTED, &priv->flags);
> > > + spin_unlock_irq(&priv->lock);
> >
> > Strange to put a lock around an atomic op like that.
> >
> > Sometimes it's valid. If another cpu was doing:
> >
> > spin_lock(lock);
> >
> > if (test_bit(IPOIB_MCAST_STARTED))
> > something();
> > ...
> > if (test_bit(IPOIB_MCAST_STARTED))
> > something_else();
> >
> > spin_unlock(lock);
> >
> > then the locked set_bit() makes sense.
> >
> > But often it doesn't ;)
>
> Good point. Michael, any reason why the lock is there around the
> set_bit()? (And similarly for the corresponding clear_bit())
>
> Thanks,
> Roland
Basically, its as Andrew said: the lock around clear_bit is there to ensure that
ipoib_mcast_send isnt running already when we stop the thread. Thats why
test_bit has to be inside the lock, too.
This was discussed with Krishna Kumar when I posted the patch originally.
For more detail, please review this thread:
http://www.mail-archive.com/[email protected]/msg13206.html
or here
http://openib.org/pipermail/openib-general/2005-December/014370.html
--
Michael S. Tsirkin
Staff Engineer, Mellanox Technologies
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