linux-kernel - Re: [PATCH v2 09/13] mm/gup: Cache *pudp in follow_pud

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Message-ID: <20240115184102.GU734935@nvidia.com>
Date: Mon, 15 Jan 2024 14:41:02 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	James Houghton <jthoughton@...gle.com>,
	David Hildenbrand <david@...hat.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Yang Shi <shy828301@...il.com>, linux-riscv@...ts.infradead.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	Rik van Riel <riel@...riel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Mike Rapoport <rppt@...nel.org>, John Hubbard <jhubbard@...dia.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michael Ellerman <mpe@...erman.id.au>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Jones <andrew.jones@...ux.dev>,
	linuxppc-dev@...ts.ozlabs.org,
	Mike Kravetz <mike.kravetz@...cle.com>,
	Muchun Song <muchun.song@...ux.dev>,
	linux-arm-kernel@...ts.infradead.org,
	Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 09/13] mm/gup: Cache *pudp in follow_pud_mask()

On Wed, Jan 03, 2024 at 05:14:19PM +0800, peterx@...hat.com wrote:
> From: Peter Xu <peterx@...hat.com>
> 
> Introduce "pud_t pud" in the function, so the code won't dereference *pudp
> multiple time.  Not only because that looks less straightforward, but also
> because if the dereference really happened, it's not clear whether there
> can be race to see different *pudp values if it's being modified at the
> same time.
> 
> Acked-by: James Houghton <jthoughton@...gle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/gup.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)

I think we have several more case like this, and I ceratinly agree
code should not access a READ_ONCE variable more than once :(

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason