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

[Peter Zijlstra]

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.. ?