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

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Message-ID: <87r09i1ou6.ffs@tglx>
Date: Tue, 17 Sep 2024 13:50:25 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Will Deacon <will@...nel.org>, "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: kernel test robot <lkp@...el.com>, Christoph Lameter via B4 Relay
 <devnull+cl.gentwo.org@...nel.org>, 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, Jani Nikula <jani.nikula@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load
 acquire

Cc+ i915 people

On Tue, Sep 17 2024 at 08:37, Will Deacon wrote:
> 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.

Why the hell can't i915 use the proper interfaces and has to bypass the
core code? Just because C allows that does not make it correct.

Can the i915 people please remove this blatant violation of layering?

Thanks,

        tglx