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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id a4-20020a544e04000000b003539686cb7bsm1758171oiy.53.2022.10.22.17.42.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 22 Oct 2022 17:42:30 -0700 (PDT) Date: Sat, 22 Oct 2022 17:42:18 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Peter Zijlstra cc: Will Deacon , x86@kernel.org, willy@infradead.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, aarcange@redhat.com, kirill.shutemov@linux.intel.com, jroedel@suse.de, ubizjak@gmail.com Subject: Re: [PATCH 07/13] mm/gup: Fix the lockless PMD access In-Reply-To: <20221022114424.906110403@infradead.org> Message-ID: <796cff9b-8eb8-8c53-9127-318d30618952@google.com> References: <20221022111403.531902164@infradead.org> <20221022114424.906110403@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 22 Oct 2022, Peter Zijlstra wrote: > On architectures where the PTE/PMD is larger than the native word size > (i386-PAE for example), READ_ONCE() can do the wrong thing. Use > pmdp_get_lockless() just like we use ptep_get_lockless(). I thought that was something Will Deacon put a lot of effort into handling around 5.8 and 5.9: see "strong prevailing wind" in include/asm-generic/rwonce.h, formerly in include/linux/compiler.h. Was it too optimistic? Did the wind drop? I'm interested in the answer, but I've certainly no objection to making this all more obviously robust - thanks. Hugh > > Signed-off-by: Peter Zijlstra (Intel) > --- > kernel/events/core.c | 2 +- > mm/gup.c | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -7186,7 +7186,7 @@ static u64 perf_get_pgtable_size(struct > return pud_leaf_size(pud); > > pmdp = pmd_offset_lockless(pudp, pud, addr); > - pmd = READ_ONCE(*pmdp); > + pmd = pmdp_get_lockless(pmdp); > if (!pmd_present(pmd)) > return 0; > > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2507,7 +2507,7 @@ static int gup_pmd_range(pud_t *pudp, pu > > pmdp = pmd_offset_lockless(pudp, pud, addr); > do { > - pmd_t pmd = READ_ONCE(*pmdp); > + pmd_t pmd = pmdp_get_lockless(pmdp); > > next = pmd_addr_end(addr, end); > if (!pmd_present(pmd))