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Thu, 18 Dec 2025 13:51:05 +0000 Message-ID: Date: Thu, 18 Dec 2025 08:51:02 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency To: David Laight Cc: Boqun Feng , Joel Fernandes , "Paul E. 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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. > > I'm not sure that example (of something that doesn't work) is really necessary. > The simple example of, given: > return a == b ? *a : 0; > the generated code might speculatively dereference 'b' (not a) before returning > zero when the pointers are different. In the past discussion that led to this new API, AFAIU, Linus made it clear that this counter example needs to be in a comment: https://lore.kernel.org/lkml/CAHk-=wgBgh5U+dyNaN=+XCdcm2OmgSRbcH4Vbtk8i5ZDGwStSA@mail.gmail.com/ This counter-example is what convinced him that this addresses a real issue. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com