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McKenney" , Will Deacon , Peter Zijlstra , Boqun Feng , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Gary Guo , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev References: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> <20240928135128.991110-2-mathieu.desnoyers@efficios.com> <02c63e79-ec8c-4d6a-9fcf-75f0e67ea242@rowland.harvard.edu> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <02c63e79-ec8c-4d6a-9fcf-75f0e67ea242@rowland.harvard.edu> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: E7EE91C0008 X-Stat-Signature: k8kmopxtq6a7aurjthn7akgmqh357sww X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1727537572-690003 X-HE-Meta: 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 iJWr5jug 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 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: On 2024-09-28 16:49, Alan Stern wrote: > On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote: [...] >> +/* >> + * Compare two addresses while preserving the address dependencies for >> + * later use of the address. It should be used when comparing an address >> + * returned by rcu_dereference(). >> + * >> + * This is needed to prevent the compiler CSE and SSA GVN optimizations >> + * from replacing the registers holding @a or @b based on their >> + * equality, which does not preserve address dependencies and allows the >> + * following misordering speculations: >> + * >> + * - If @b is a constant, the compiler can issue the loads which depend >> + * on @a before loading @a. >> + * - If @b is a register populated by a prior load, weakly-ordered >> + * CPUs can speculate loads which depend on @a before loading @a. >> + * >> + * The same logic applies with @a and @b swapped. [...] >> + * >> + * Return value: true if pointers are equal, false otherwise. >> + * >> + * The compiler barrier() is ineffective at fixing this issue. It does >> + * not prevent the compiler CSE from losing the address dependency: >> + * >> + * int fct_2_volatile_barriers(void) >> + * { >> + * int *a, *b; >> + * >> + * do { >> + * a = READ_ONCE(p); >> + * asm volatile ("" : : : "memory"); >> + * b = READ_ONCE(p); >> + * } while (a != b); >> + * asm volatile ("" : : : "memory"); <-- barrier() >> + * return *b; >> + * } >> + * >> + * With gcc 14.2 (arm64): >> + * >> + * fct_2_volatile_barriers: >> + * adrp x0, .LANCHOR0 >> + * add x0, x0, :lo12:.LANCHOR0 >> + * .L2: >> + * ldr x1, [x0] <-- x1 populated by first load. >> + * ldr x2, [x0] >> + * cmp x1, x2 >> + * bne .L2 >> + * ldr w0, [x1] <-- x1 is used for access which should depend on b. >> + * ret >> + * >> + * On weakly-ordered architectures, this lets CPU speculation use the >> + * result from the first load to speculate "ldr w0, [x1]" before >> + * "ldr x2, [x0]". >> + * Based on the RCU documentation, the control dependency does not >> + * prevent the CPU from speculating loads. > > IMO, this lengthy explanation is not needed in the source code. Just > refer interested readers to the commit description. You're repeating > the same text verbatim, after all. > > (Or if you firmly believe that this explanation _does_ belong in the > code, then omit it from the commit description. There's no need to say > everything twice.) > Linus asked for this code/asm example to be in the comment: https://lore.kernel.org/lkml/CAHk-=wgBgh5U+dyNaN=+XCdcm2OmgSRbcH4Vbtk8i5ZDGwStSA@mail.gmail.com/ I agree that it may be a bit verbose for the comment. Linus, do you want the explanation wrt compiler barriers not being enough (with C/asm example) in the comment above ptr_eq() or in the commit message ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com