Re: [PATCH] Bug #20116: Clarify barrier-like and mutex-like behaviours of PD->lock.

[Florian Weimer <fweimer at redhat dot com>]

On 02/13/2017 07:49 PM, Florian Weimer wrote:
> On 02/13/2017 02:29 PM, Carlos O'Donell wrote:
> > +   It is important to point out that PD->lock is being used as a POSIX
> > +   barrier and a POSIX mutex.  The lock is taken in the parent to force
> > +   the child to wait, and then the child releases the lock, in effect a
> > +   barrier.  However, this barrier-like effect is used only for
> > +   synchronizing the parent and child.  After startup the lock is used
> > +   like a mutex to create a critical region during which a single owner
> > +   modifies the thread parameters.
>
> I had missed that the lock was reused for the scheduler parameter.
>
> But the current code still does not make sense to me.  Why do we need to
> keep a copy of the scheduler parameters at all?  Is this just a cache to
> improve performance, similar to what we used to do for the PID?

The cache is used in the implementation of PTHREAD_PRIO_PROTECT mutexes. 
 There are data races:

  https://sourceware.org/bugzilla/show_bug.cgi?id=21160

I expect that the use of ->lock to protect these members will go away 
eventually.

Thanks,
Florian