include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+)
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 <torvalds@linux-foundation.org>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: John Stultz <jstultz@google.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: maged.michael@gmail.com
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: rcu@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: lkmm@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;
+}
+
#define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
/**
--
2.39.2[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 <torvalds@linux-foundation.org>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: John Stultz <jstultz@google.com>
> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: maged.michael@gmail.com
> Cc: Mateusz Guzik <mjguzik@gmail.com>
> Cc: rcu@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: lkmm@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.feng@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
>
On 2024-09-27 22:33, Mathieu Desnoyers wrote:
[...]
> ---
> include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 62 insertions(+)
>
I'm wondering if this really belongs in compiler.h, or if it's so
RCU/HP specific that it should be implemented in rcupdate.h ?
[... ]
> +static __always_inline
> +int ptr_eq(const volatile void *a, const volatile void *b)
And perhaps rename this to rcu_ptr_eq() ?
Thanks,
Mathieu
> +{
> + OPTIMIZER_HIDE_VAR(a);
> + OPTIMIZER_HIDE_VAR(b);
> + return a == b;
> +}
> +
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
On Fri, Sep 27, 2024 at 07:05:53PM -0400, Mathieu Desnoyers wrote:
> On 2024-09-27 22:33, Mathieu Desnoyers wrote:
> [...]
>
> > ---
> > include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 62 insertions(+)
> >
>
> I'm wondering if this really belongs in compiler.h, or if it's so
> RCU/HP specific that it should be implemented in rcupdate.h ?
>
> [... ]
> > +static __always_inline
> > +int ptr_eq(const volatile void *a, const volatile void *b)
>
> And perhaps rename this to rcu_ptr_eq() ?
For either location or name:
Acked-by: Paul E. McKenney <paulmck@kernel.org>
There are non-RCU uses, but RCU is currently by far the most common.
Thanx, Paul
> Thanks,
>
> Mathieu
>
> > +{
> > + OPTIMIZER_HIDE_VAR(a);
> > + OPTIMIZER_HIDE_VAR(b);
> > + return a == b;
> > +}
> > +
>
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>
On Fri, Sep 27, 2024 at 07:05:53PM -0400, Mathieu Desnoyers wrote:
> On 2024-09-27 22:33, Mathieu Desnoyers wrote:
> [...]
>
> > ---
> > include/linux/compiler.h | 62 ++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 62 insertions(+)
> >
>
> I'm wondering if this really belongs in compiler.h, or if it's so
> RCU/HP specific that it should be implemented in rcupdate.h ?
>
> [... ]
> > +static __always_inline
> > +int ptr_eq(const volatile void *a, const volatile void *b)
>
> And perhaps rename this to rcu_ptr_eq() ?
>
I think the current place and name is fine, yes RCU is the most
important address dependency user, but there are other users as well.
Regards,
Boqun
> Thanks,
>
> Mathieu
>
> > +{
> > + OPTIMIZER_HIDE_VAR(a);
> > + OPTIMIZER_HIDE_VAR(b);
> > + return a == b;
> > +}
> > +
>
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>
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