[Cc Gary]
On Fri, Sep 27, 2024 at 04:33:34PM -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 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.
>
> 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.
>
> Suggested-by: Linus Torvalds <torva...@linux-foundation.org>
> Suggested-by: Boqun Feng <boqun.f...@gmail.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Cc: "Paul E. McKenney" <paul...@kernel.org>
> Cc: Will Deacon <w...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Boqun Feng <boqun.f...@gmail.com>
> Cc: Alan Stern <st...@rowland.harvard.edu>
> Cc: John Stultz <jstu...@google.com>
> Cc: Neeraj Upadhyay <neeraj.upadh...@amd.com>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Boqun Feng <boqun.f...@gmail.com>
> Cc: Frederic Weisbecker <frede...@kernel.org>
> Cc: Joel Fernandes <j...@joelfernandes.org>
> Cc: Josh Triplett <j...@joshtriplett.org>
> Cc: Uladzislau Rezki <ure...@gmail.com>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Lai Jiangshan <jiangshan...@gmail.com>
> Cc: Zqiang <qiang.zhang1...@gmail.com>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Waiman Long <long...@redhat.com>
> Cc: Mark Rutland <mark.rutl...@arm.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Vlastimil Babka <vba...@suse.cz>
> Cc: maged.mich...@gmail.com
> Cc: Mateusz Guzik <mjgu...@gmail.com>
> Cc: r...@vger.kernel.org
> Cc: linux...@kvack.org
> Cc: l...@lists.linux.dev
> ---
> include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 62 insertions(+)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 2df665fa2964..f26705c267e8 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -186,6 +186,68 @@ 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 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.
> + */
> +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;
> +}
> +
This is better than what I proposed, thank you!
Reviewed-by: Boqun Feng <boqun.f...@gmail.com>
Could you also make some documentation changes at the "Be very careful
about comparing pointers obtained from..." paragraph in
Documentation/RCU/rcu_dereference.rst? Since 'ptr_eq' is a good tool in
those cases mentioned there. Thanks.
Regards,
Boqun
> #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix),
> __COUNTER__)
>
> /**
> --
> 2.39.2
>
