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Date: Fri, 12 Oct 2018 16:56:55 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm/kasan: make quarantine_lock a raw_spinlock_t
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To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Dmitry Vyukov <dvyukov@google.com>, Clark Williams <williams@redhat.com>, Alexander Potapenko <glider@google.com>, kasan-dev <kasan-dev@googlegroups.com>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, linux-rt-users@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>

On Wed, 10 Oct 2018 23:49:45 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> On 2018-10-10 11:57:41 [+0200], Dmitry Vyukov wrote:
> > Yes. Clark's patch looks good to me. Probably would be useful to add a
> > comment as to why raw spinlock is used (otherwise somebody may
> > refactor it back later).
> 
> If you really insist, I could add something but this didn't happen so
> far. git's changelog should provide enough information why to why it was
> changed.

Requiring code readers to look up changelogs in git is rather user-hostile.
There are several reasons for using raw_*, so an explanatory comment at
each site is called for.

However it would be smarter to stop "using raw_* for several reasons". 
Instead, create a differently named variant for each such reason.  ie, do

/*
 * Nice comment goes here.  It explains all the possible reasons why -rt
 * might use a raw_spin_lock when a spin_lock could otherwise be used.
 */
#define raw_spin_lock_for_rt	raw_spinlock

Then use raw_spin_lock_for_rt() at all such sites.