linux-kernel - Re: [PATCH] riscv: Use an atomic xchg in pudp_huge_get_and

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Message-ID: <bfffde06-5d4c-4e2c-adfe-e48590bf2f3d@ghiti.fr>
Date: Mon, 22 Sep 2025 08:19:55 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Paul Walmsley <pjw@...nel.org>, Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] riscv: Use an atomic xchg in pudp_huge_get_and_clear()

On 9/20/25 03:39, Paul Walmsley wrote:
> Hi Alex,
>
> On Thu, 14 Aug 2025, Alexandre Ghiti wrote:
>
>> Make sure we return the right pud value and not a value that could
>> have been overwritten in between by a different core.
>>
>> Fixes: c3cc2a4a3a23 ("riscv: Add support for PUD THP")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
>> ---
>> Note that this will conflict with
>> https://lore.kernel.org/linux-riscv/20250625063753.77511-1-ajd@linux.ibm.com/
>> if applied after 6.17.
> Two quick questions on this one:
>
> - I see that you're using atomic_long_xchg() here and in some similar
> functions in pgtable.h, rather than xchg().  Was curious about the
> rationale for that?


Both functions amount to the same, I just used the same function as for 
existing similar functions.


>
> - x86 avoids the xchg() for !CONFIG_SMP.  Should we do the same?


Sounds like micro optimization to me, but up to you.


>
> thanks,
>
> - Paul
>