Re: [PATCH v2 2/6] sched/cpuset: Bring back cpuset_mutex

[Juri Lelli]

On 04/05/23 08:18, Peter Zijlstra wrote:
> On Wed, May 03, 2023 at 09:22:24AM +0200, Juri Lelli wrote:
> 
> >  /*
> > - * There are two global locks guarding cpuset structures - cpuset_rwsem and
> > + * There are two global locks guarding cpuset structures - cpuset_mutex and
> >   * callback_lock. We also require taking task_lock() when dereferencing a
> >   * task's cpuset pointer. See "The task_lock() exception", at the end of this
> > - * comment.  The cpuset code uses only cpuset_rwsem write lock.  Other
> > - * kernel subsystems can use cpuset_read_lock()/cpuset_read_unlock() to
> > - * prevent change to cpuset structures.
> > + * comment.  The cpuset code uses only cpuset_mutex. Other kernel subsystems
> > + * can use cpuset_lock()/cpuset_unlock() to prevent change to cpuset
> > + * structures.
> >   *
> >   * A task must hold both locks to modify cpusets.  If a task holds
> > - * cpuset_rwsem, it blocks others wanting that rwsem, ensuring that it
> > - * is the only task able to also acquire callback_lock and be able to
> > - * modify cpusets.  It can perform various checks on the cpuset structure
> > - * first, knowing nothing will change.  It can also allocate memory while
> > - * just holding cpuset_rwsem.  While it is performing these checks, various
> > - * callback routines can briefly acquire callback_lock to query cpusets.
> > - * Once it is ready to make the changes, it takes callback_lock, blocking
> > - * everyone else.
> > + * cpuset_mutex, it blocks others, ensuring that it is the only task able to
> > + * also acquire callback_lock and be able to modify cpusets.  It can perform
> > + * various checks on the cpuset structure first, knowing nothing will change.
> > + * It can also allocate memory while just holding cpuset_mutex.  While it is
> > + * performing these checks, various callback routines can briefly acquire
> > + * callback_lock to query cpusets.  Once it is ready to make the changes, it
> > + * takes callback_lock, blocking everyone else.
> >   *
> >   * Calls to the kernel memory allocator can not be made while holding
> >   * callback_lock, as that would risk double tripping on callback_lock
> > @@ -403,16 +402,16 @@ static struct cpuset top_cpuset = {
> >   * guidelines for accessing subsystem state in kernel/cgroup.c
> >   */
> >  
> > -DEFINE_STATIC_PERCPU_RWSEM(cpuset_rwsem);
> > +static DEFINE_MUTEX(cpuset_mutex);
> 
> Perhaps extend the comment to state you explicitly want a mutex for PI
> etc.. ?
> 

Sure, can do that.

Thanks!