linux-kernel - Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <20250115104841.GX5388@noisy.programming.kicks-ass.net>
Date: Wed, 15 Jan 2025 11:48:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Mateusz Guzik <mjguzik@...il.com>, akpm@...ux-foundation.org,
	willy@...radead.org, liam.howlett@...cle.com,
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Subject: Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a
 reference count

On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote:

> > Replacing down_read_trylock() with the new routine loses an acquire
> > fence. That alone is not a problem, but see below.
> 
> Hmm. I think this acquire fence is actually necessary. We don't want
> the later vm_lock_seq check to be reordered and happen before we take
> the refcount. Otherwise this might happen:
> 
> reader             writer
> if (vm_lock_seq == mm_lock_seq) // check got reordered
>         return false;
>                        vm_refcnt += VMA_LOCK_OFFSET
>                        vm_lock_seq == mm_lock_seq
>                        vm_refcnt -= VMA_LOCK_OFFSET
> if (!__refcount_inc_not_zero_limited())
>         return false;
> 
> Both reader's checks will pass and the reader would read-lock a vma
> that was write-locked.

Hmm, you're right. That acquire does matter here.