linux-kernel - Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit

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Message-ID: <YMsLtseEHC8dWwag@hirez.programming.kicks-ass.net>
Date:   Thu, 17 Jun 2021 10:45:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit

On Wed, Jun 16, 2021 at 11:41:19AM -0700, Andy Lutomirski wrote:
> mmgrab() and mmdrop() would be better if they were not full barriers.  As a
> trivial optimization,

> mmgrab() could use a relaxed atomic and mmdrop()
> could use a release on architectures that have these operations.

mmgrab() *is* relaxed, mmdrop() is a full barrier but could trivially be
made weaker once membarrier stops caring about it.

static inline void mmdrop(struct mm_struct *mm)
{
	unsigned int val = atomic_dec_return_release(&mm->mm_count);
	if (unlikely(!val)) {
		/* Provide REL+ACQ ordering for free() */
		smp_acquire__after_ctrl_dep();
		__mmdrop(mm);
	}
}

It's slightly less optimal for not being able to use the flags from the
decrement. Or convert the whole thing to refcount_t (if appropriate)
which already does something like the above.