On Wed, 17 Dec 2025 20:45:28 -0500 Mathieu Desnoyers <[email protected]> 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. > > Suggested-by: Linus Torvalds <[email protected]> > Suggested-by: Boqun Feng <[email protected]> > Signed-off-by: Mathieu Desnoyers <[email protected]> > Reviewed-by: Boqun Feng <[email protected]> > Reviewed-by: Joel Fernandes (Google) <[email protected]> > Tested-by: Joel Fernandes (Google) <[email protected]> > Acked-by: "Paul E. McKenney" <[email protected]> > Acked-by: Alan Stern <[email protected]> > Cc: Greg Kroah-Hartman <[email protected]> > Cc: Sebastian Andrzej Siewior <[email protected]> > Cc: "Paul E. McKenney" <[email protected]> > Cc: Will Deacon <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Boqun Feng <[email protected]> > Cc: Alan Stern <[email protected]> > Cc: John Stultz <[email protected]> > Cc: Neeraj Upadhyay <[email protected]> > Cc: Linus Torvalds <[email protected]> > Cc: Boqun Feng <[email protected]> > Cc: Frederic Weisbecker <[email protected]> > Cc: Joel Fernandes <[email protected]> > Cc: Josh Triplett <[email protected]> > Cc: Uladzislau Rezki <[email protected]> > Cc: Steven Rostedt <[email protected]> > Cc: Lai Jiangshan <[email protected]> > Cc: Zqiang <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Waiman Long <[email protected]> > Cc: Mark Rutland <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Vlastimil Babka <[email protected]> > Cc: [email protected] > Cc: Mateusz Guzik <[email protected]> > Cc: Gary Guo <[email protected]> > Cc: Jonas Oberhauser <[email protected]> > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: Nikita Popov <[email protected]> > Cc: [email protected] > --- > Changes since v0: > - Include feedback from Alan Stern. > --- > include/linux/compiler.h | 63 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 63 insertions(+) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 5b45ea7dff3e..c5ca3b54c112 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -163,6 +163,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f, > int val, > __asm__ ("" : "=r" (var) : "0" (var)) > #endif > > +/* > + * 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 > + * > + * 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. David > + */ > +static __always_inline > +int ptr_eq(const volatile void *a, const volatile void *b) > +{ > + OPTIMIZER_HIDE_VAR(a); > + OPTIMIZER_HIDE_VAR(b); > + return a == b; > +} > + > #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), > __COUNTER__) > > /**
