On Tue, Oct 01, 2024 at 09:02:02PM -0400, Mathieu Desnoyers wrote: > Compiler CSE and SSA GVN optimizations can cause the address dependency > of addresses returned by rcu_dereference to be lost when comparing those > pointers with either constants or previously loaded pointers. > > Introduce ptr_eq() to 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 using @a (or @b) in places where the source refers to @b (or @a) > based on the fact that after the comparison, the two are known to be > equal, 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. > [...] > +/* > + * 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 using @a (or @b) in places where the source refers to @b (or @a) > + * based on the fact that after the comparison, the two are known to be > + * equal, 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 > + *
I could reproduce this in compiler explorer, but I'm curious what flags are you using? For me it does a bunch of usage of the stack for temporary storage (still incorrectly returns *a though as you pointed). Interestingly, if I just move the comparison into an an __always_inline__ function like below, but without the optimizer hide stuff, gcc 14.2 on arm64 does generate the correct code: static inline __attribute__((__always_inline__)) int ptr_eq(const volatile void *a, const volatile void *b) { /* No OPTIMIZER_HIDE_VAR */ return a == b; } volatile int *p = 0; int fct_2_volatile_barriers() { int *a, *b; do { a = READ_ONCE(p); asm volatile ("" : : : "memory"); b = READ_ONCE(p); } while (!ptr_eq(a, b)); asm volatile ("" : : : "memory"); // barrier() return *b; } But not sure if it fixes the speculation issue you referred to. Putting back the OPTIMIZER_HIDE_VAR() then just seems to pass the a and b stored on the stack through a washing machine: ldr x0, [sp, 8] str x0, [sp, 8] ldr x0, [sp] str x0, [sp] And here I thought the "" in OPTIMIZER_HIDE_VAR was not supposed to generate any code but I guess it is still a NOOP. Anyway, as such this LGTM since whether OPTIMIZER_HIDE_VAR() used or not, it does fix the problem. Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org> thanks, - Joel