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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 ARC-Authentication-Results: i=1; imf25.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=X1vMU61X; spf=pass (imf25.hostedemail.com: domain of hughd@google.com designates 209.85.167.179 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666485752; a=rsa-sha256; cv=none; b=JgGl2BC+oenrMIJPmimeUJu3ixYMS2vd76niA9hVtlcZL4SBWmjaJYwfTDoVxYT7+scodW Yub8XmZkUra/E+hV6rjL5sgNl6z7ZJtjZt9EYXRVIhU7HvRzquJhdfG2syP8wqsDf/GSrI 6MKsK8llj8zLTRFutN17vYadYnraA0w= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666485752; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=cPzp800k6N3WRemPfpaZ33nVr+8Go0OaG+uQGTl/BQU=; b=Bug8+jt0fPBRN7U2fUMJ5a8Qz8M2agHyAzcGt+fKIrp5l2ElpItnZZKKSzL5HR0DztqFdv CiLTsx9aStbqIqWlzBLw0ZKAT6EFBtOHjR5AUIIwEgfkS0HeLDNccw2b6lKw+MKdtEl6jz nqa1E6jk2NP5TevqjqFhpJHZqLlKGSE= Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=X1vMU61X; spf=pass (imf25.hostedemail.com: domain of hughd@google.com designates 209.85.167.179 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspam-User: X-Rspamd-Server: rspam10 X-Stat-Signature: nhgyzobb6199f9ycj19srgoh38c6yw9n X-Rspamd-Queue-Id: 5DFAAA0027 X-HE-Tag: 1666485752-180085 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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))