linux-kernel - Re: [PATCH v3] Avoid memory barrier in read

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Message-ID: <20240917073703.GB27290@willie-the-truck>
Date: Tue, 17 Sep 2024 08:37:04 +0100
From: Will Deacon <will@...nel.org>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: kernel test robot <lkp@...el.com>,
	Christoph Lameter via B4 Relay <devnull+cl.gentwo.org@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>, oe-kbuild-all@...ts.linux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load
 acquire

On Mon, Sep 16, 2024 at 10:52:18AM -0700, Christoph Lameter (Ampere) wrote:
> On Fri, 13 Sep 2024, kernel test robot wrote:
> 
> > >> drivers/gpu/drm/i915/gt/intel_tlb.h:21:47: error: macro "seqprop_sequence" requires 2 arguments, but only 1 given
> 
> From 15d86bc9589f16947c5fb0f34d2947eacd48f853 Mon Sep 17 00:00:00 2001
> From: Christoph Lameter <cl@...two.org>
> Date: Mon, 16 Sep 2024 10:44:16 -0700
> Subject: [PATCH] Update Intel DRM use of seqprop_sequence
> 
> One of Intels drivers uses seqprop_sequence() for its tlb sequencing.
> We added a parameter so that we can use acquire. Its pretty safe to
> assume that this will work without acquire.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_tlb.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.h b/drivers/gpu/drm/i915/gt/intel_tlb.h
> index 337327af92ac..81998c4cd4fb 100644
> --- a/drivers/gpu/drm/i915/gt/intel_tlb.h
> +++ b/drivers/gpu/drm/i915/gt/intel_tlb.h
> @@ -18,7 +18,7 @@ void intel_gt_fini_tlb(struct intel_gt *gt);
> 
>  static inline u32 intel_gt_tlb_seqno(const struct intel_gt *gt)
>  {
> -	return seqprop_sequence(&gt->tlb.seqno);
> +	return seqprop_sequence(&gt->tlb.seqno, false);
>  }

Yikes, why is the driver using the seqlock internals here? It's a bit of
a pity, as a quick grep suggest that this is the _only_ user of
'seqcount_mutex_t', yet it's still having to work around the API.

Will