Re: [PATCH] powerpc/64s/radix: reset mm_cpumask for single thread process when possible

On Wed, 2018-05-09 at 16:56 +1000, Nicholas Piggin wrote:

When a single-threaded process has a non-local mm_cpumask and requires
a full PID tlbie invalidation, use that as an opportunity to reset the
cpumask back to the current CPU we're running on.

No other thread can concurrently switch to this mm, because it must
have had a reference on mm_users before it could use_mm. mm_users can
be asynchronously incremented e.g., by mmget_not_zero, but those users
must not be doing use_mm.

What do you mean ? I don't fully understand how this isn't racy with
another thread being created, switching to that mm, and then having its
bit cleared by us ? Also why not use_mm ? what prevents it ?

Cheers,
Ben.