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Message-ID: <20241003000843.GA192403@google.com>
Date: Thu, 3 Oct 2024 00:08:43 +0000
From: Joel Fernandes <joel@...lfernandes.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
Nicholas Piggin <npiggin@...il.com>,
Michael Ellerman <mpe@...erman.id.au>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
"Paul E. McKenney" <paulmck@...nel.org>,
Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
Alan Stern <stern@...land.harvard.edu>,
John Stultz <jstultz@...gle.com>,
Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
Frederic Weisbecker <frederic@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Uladzislau Rezki <urezki@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar <mingo@...hat.com>,
Waiman Long <longman@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
Mateusz Guzik <mjguzik@...il.com>,
Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev,
Gary Guo <gary@...yguo.net>, Nikita Popov <github@...pov.com>,
llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 1/4] compiler.h: Introduce ptr_eq() to preserve
address dependency
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) <joel@...lfernandes.org>
thanks,
- Joel
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