Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

Paul E. McKenney
2014-06-18T17:00:46Z

On Wed, Jun 18, 2014 at 06:43:59PM +0200, Oleg Nesterov wrote:
> On 06/17, Paul E. McKenney wrote:
> >
> > + if (drop_boost_mutex) {
> > + rt_mutex_unlock(&rnp->boost_mtx);
> > complete(&rnp->boost_completion);
>
> Well, I still do not understand this ->boost_completion...
>
> > - /* Wait until boostee is done accessing mtx before reinitializing. */
> > + /* Wait for boostee to be done w/boost_mtx before reinitializing. */
> > wait_for_completion(&rnp->boost_completion);
>
> OK, at least we have a comment.
>
> But let me repeat. Thomas has already fixed rt_mutex, unlock is atomic.
> It doesn't touch this memory after it makes another lock() possible.
>
> And (contrary to what I said initially) we can rely on this because -rt
> converts spinlock_t into rt_mutex ?

Well, perhaps I should rein in my paranoia on this one. That said, the
cost of my paranoia is minimal in this slowpath.

Thanx, Paul