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 Wed, 9 May 2018 09:32:01 +0100
Subject: Re: [PATCH] powerpc/64s/radix: reset mm_cpumask for single thread
 process when possible
From: Benjamin Herrenschmidt <benh@au1.ibm.com>
Reply-To: benh@au1.ibm.com
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
Date: Wed, 09 May 2018 18:31:55 +1000
In-Reply-To: <20180509065613.14762-1-npiggin@gmail.com>
References: <20180509065613.14762-1-npiggin@gmail.com>
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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.