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Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org> <1ca9f99f-6266-47ca-8c94-1a9b9aaa717f@kernel.org> <37973e21-e8f4-4603-b93d-4e0b1b2499fa@lucifer.local> <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> From: "Christophe Leroy (CS GROUP)" Content-Language: fr-FR In-Reply-To: <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 74C11A0005 X-Rspamd-Server: rspam11 X-Stat-Signature: f9bhdkrmsdnx948u867tczfj538gyk8o X-HE-Tag: 1764272666-636715 X-HE-Meta: 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 B0lpfV4g hHJWUsmsqvb4m8VOX3M9kmKJ5PzAcGuFjRLeVkN6vxhJDWW5uD1roPrM6a4mj+g18dJXfnSbJM8UORAPyS1SEvvXU+SX8Si+v5/1FA6WGWMwyJeFkHOo84Xr+y2QE+TcmNQOteA4U96oBO0nV4BuScCG4ZJidbA+qc3RwNncYlDJ9Cmni/DuzkHWZ12zt5LXtL1QRJcAHLLIJSg0o2u7rbYvxHhYUKC9jBv3jsc5WkRiWGrQQCQ4/IU5DDo/BG6VFANipbp/g6/7AUdja24hHiN2ZS3OEu2ub46epRCcG5tzS3LsMrCFuL60ZX1lwyjPiCRUR 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: List-Subscribe: List-Unsubscribe: Le 26/11/2025 à 17:34, Ryan Roberts a écrit : > On 26/11/2025 16:07, Ryan Roberts wrote: >> On 26/11/2025 15:12, David Hildenbrand (Red Hat) wrote: >>> On 11/26/25 16:08, Lorenzo Stoakes wrote: >>>> On Wed, Nov 26, 2025 at 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote: >>>>> On 11/26/25 15:52, Lorenzo Stoakes wrote: >>>>>> >>>>>> Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up >>>>>> requiring a READ_ONCE() further up the stack? >>>>> >>>>> See my other reply, I think the pmdp_get() is required because all pud_* >>>>> functions are just simple stubs. >>>> >>>> OK, thought you were saying we should push further down the stack? Or up >>>> depending on how you view these things :P as in READ_ONCE at leaf? >>> >>> I think at leaf because I think the previous ones should essentially be only >>> used by stubs. >>> >>> But I haven't fully digested how this is all working. Or supposed to work. >>> >>> I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h example to >>> see if I can make sense of it, >> >> I wonder if we can think about this slightly differently; >> >> READ_ONCE() has two important properties: >> >> - It guarrantees that a load will be issued, *even if output is unused* >> - It guarrantees that the read will be single-copy-atomic (no tearing) >> >> I think for the existing places where READ_ONCE() is used for pagetable reads we >> only care about: >> >> - It guarrantees that a load will be issued, *if output is used* >> - It guarrantees that the read will be single-copy-atomic (no tearing) >> >> I think if we can weaken to the "if output is used" property, then the compiler >> will optimize out all the unneccessary reads. >> >> AIUI, a C dereference provides neither of the guarrantees so that's no good. >> >> What about non-volatile asm? I'm told (thought need to verify) that for >> non-volatile asm, the compiler will emit it if the output is used and remove it >> otherwise. So if the asm contains the required single-copy-atomic, perhaps we >> are in business? >> >> So we would need a new READ_SCA() macro that could default to READ_ONCE() (which >> is stronger) and arches could opt in to providing a weaker asm version. Then the >> default pXdp_get() could be READ_SCA(). And this should work for all cases. >> >> I think. > > I'm not sure this works. It looks like the compiler is free to move non-volatile > asm sections which might be problematic for places where we are currently using > READ_ONCE() in lockless algorithms, (e.g. GUP?). We wouldn't want to end up with > a stale value. What about adding a memory clobber to the non-volatile asm ? Compiler shouldn't move the asm section in that case. Christophe