Re: [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks

From: Uros Bizjak
Date: Tue Apr 04 2023 - 08:25:48 EST

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On Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote:
> > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > >
> > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote:
> > > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote:
> > > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > > > >
> > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote:
> > > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning.
> > > > > >
> > > > > > Can you give an example of where we are passing an incompatible pointer?
> > > > >
> > > > > An example is patch 10/10 from the series, which will fail without
> > > > > this fix when fallback code is used. We have:
> > > > >
> > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset);
> > > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head));
> > > > >
> > > > > where rb->head is defined as:
> > > > >
> > > > > typedef struct {
> > > > > atomic_long_t a;
> > > > > } local_t;
> > > > >
> > > > > while offset is defined as 'unsigned long'.
> > > >
> > > > Ok, but that's because we're doing the wrong thing to start with.
> > > >
> > > > Since local_t is defined in terms of atomic_long_t, we should define the
> > > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still
> > > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix
> > > > that in the callsite:
> > > >
> > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head))
> > >
> > > Sorry, that should be:
> > >
> > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head))
> >
> > The fallbacks are a bit more complicated than above, and are different
> > from atomic_try_cmpxchg.
> >
> > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local
> > are not defined call arch_cmpxchg_local. Also in patch 2/10,
> > try_cmpxchg_local is introduced, where it calls
> > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g.
> > :
> >
> > #define local_cmpxchg(l, o, n) \
> > (cmpxchg_local(&((l)->a.counter), (o), (n)))
> > +#define local_try_cmpxchg(l, po, n) \
> > + (try_cmpxchg_local(&((l)->a.counter), (po), (n)))
> >
> > which is part of the local_t API. Targets should either define all
> > these #defines, or none. There are no partial fallbacks as is the case
> > with atomic_t.
>
> Whether or not there are fallbacks is immaterial.
>
> In those cases, architectures can just as easily write C wrappers, e.g.
>
> long local_cmpxchg(local_t *l, long old, long new)
> {
> return cmpxchg_local(&l->a.counter, old, new);
> }
>
> long local_try_cmpxchg(local_t *l, long *old, long new)
> {
> return try_cmpxchg_local(&l->a.counter, old, new);
> }

Please find attached the complete prototype patch that implements the
above suggestion.

The patch includes:
- implementation of instrumented try_cmpxchg{,64}_local definitions
- corresponding arch_try_cmpxchg{,64}_local fallback definitions
- generic local{,64}_try_cmpxchg (and local{,64}_cmpxchg) C wrappers

- x86 specific local_try_cmpxchg (and local_cmpxchg) C wrappers
- x86 specific arch_try_cmpxchg_local definition

- kernel/events/ring_buffer.c change to test local_try_cmpxchg
implementation and illustrate the transition
- arch/x86/events/core.c change to test local64_try_cmpxchg
implementation and illustrate the transition

The definition of atomic_long_t is different for 64-bit and 32-bit
targets (s64 vs int), so target specific C wrappers have to use
different casts to account for this difference.

Uros.
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index d096b04bf80e..d9310e9363f1 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -129,13 +129,12 @@ u64 x86_perf_event_update(struct perf_event *event)
* exchange a new raw count - then add that new-prev delta
* count to the generic event atomically:
*/
-again:
prev_raw_count = local64_read(&hwc->prev_count);
- rdpmcl(hwc->event_base_rdpmc, new_raw_count);

- if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
- new_raw_count) != prev_raw_count)
- goto again;
+ do {
+ rdpmcl(hwc->event_base_rdpmc, new_raw_count);
+ } while (!local64_try_cmpxchg(&hwc->prev_count, &prev_raw_count,
+ new_raw_count));

/*
* Now we have the new raw value and have updated the prev
diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
index 94fbe6ae7431..540573f515b7 100644
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -221,9 +221,15 @@ extern void __add_wrong_size(void)
#define __try_cmpxchg(ptr, pold, new, size) \
__raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX)

+#define __try_cmpxchg_local(ptr, pold, new, size) \
+ __raw_try_cmpxchg((ptr), (pold), (new), (size), "")
+
#define arch_try_cmpxchg(ptr, pold, new) \
__try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))

+#define arch_try_cmpxchg_local(ptr, pold, new) \
+ __try_cmpxchg_local((ptr), (pold), (new), sizeof(*(ptr)))
+
/*
* xadd() adds "inc" to "*ptr" and atomically returns the previous
* value of "*ptr".
diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 349a47acaa4a..d286a6c7c0b7 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -120,8 +120,20 @@ static inline long local_sub_return(long i, local_t *l)
#define local_inc_return(l) (local_add_return(1, l))
#define local_dec_return(l) (local_sub_return(1, l))

-#define local_cmpxchg(l, o, n) \
- (cmpxchg_local(&((l)->a.counter), (o), (n)))
+static inline long local_cmpxchg(local_t *l, long old, long new)
+{
+ return cmpxchg_local(&l->a.counter, old, new);
+}
+
+static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
+{
+#ifdef CONFIG_64BIT
+ return try_cmpxchg_local(&l->a.counter, (s64 *)old, new);
+#else
+ return try_cmpxchg_local(&l->a.counter, (int *)old, new);
+#endif
+}
+
/* Always has a lock prefix */
#define local_xchg(l, n) (xchg(&((l)->a.counter), (n)))

diff --git a/include/asm-generic/local.h b/include/asm-generic/local.h
index fca7f1d84818..7f97018df66f 100644
--- a/include/asm-generic/local.h
+++ b/include/asm-generic/local.h
@@ -42,6 +42,7 @@ typedef struct
#define local_inc_return(l) atomic_long_inc_return(&(l)->a)

#define local_cmpxchg(l, o, n) atomic_long_cmpxchg((&(l)->a), (o), (n))
+#define local_try_cmpxchg(l, po, n) atomic_long_try_cmpxchg((&(l)->a), (po), (n))
#define local_xchg(l, n) atomic_long_xchg((&(l)->a), (n))
#define local_add_unless(l, _a, u) atomic_long_add_unless((&(l)->a), (_a), (u))
#define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a)
diff --git a/include/asm-generic/local64.h b/include/asm-generic/local64.h
index 765be0b7d883..14963a7a6253 100644
--- a/include/asm-generic/local64.h
+++ b/include/asm-generic/local64.h
@@ -42,7 +42,16 @@ typedef struct {
#define local64_sub_return(i, l) local_sub_return((i), (&(l)->a))
#define local64_inc_return(l) local_inc_return(&(l)->a)

-#define local64_cmpxchg(l, o, n) local_cmpxchg((&(l)->a), (o), (n))
+static inline s64 local64_cmpxchg(local64_t *l, s64 old, s64 new)
+{
+ return local_cmpxchg(&l->a, old, new);
+}
+
+static inline bool local64_try_cmpxchg(local64_t *l, s64 *old, s64 new)
+{
+ return local_try_cmpxchg(&l->a, (long *)old, new);
+}
+
#define local64_xchg(l, n) local_xchg((&(l)->a), (n))
#define local64_add_unless(l, _a, u) local_add_unless((&(l)->a), (_a), (u))
#define local64_inc_not_zero(l) local_inc_not_zero(&(l)->a)
@@ -81,6 +90,7 @@ typedef struct {
#define local64_inc_return(l) atomic64_inc_return(&(l)->a)

#define local64_cmpxchg(l, o, n) atomic64_cmpxchg((&(l)->a), (o), (n))
+#define local64_try_cmpxchg(l, po, n) atomic64_try_cmpxchg((&(l)->a), (po), (n))
#define local64_xchg(l, n) atomic64_xchg((&(l)->a), (n))
#define local64_add_unless(l, _a, u) atomic64_add_unless((&(l)->a), (_a), (u))
#define local64_inc_not_zero(l) atomic64_inc_not_zero(&(l)->a)
diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h
index 77bc5522e61c..36c92851cdee 100644
--- a/include/linux/atomic/atomic-arch-fallback.h
+++ b/include/linux/atomic/atomic-arch-fallback.h
@@ -217,6 +217,28 @@

#endif /* arch_try_cmpxchg64_relaxed */

+#ifndef arch_try_cmpxchg_local
+#define arch_try_cmpxchg_local(_ptr, _oldp, _new) \
+({ \
+ typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \
+ ___r = arch_cmpxchg_local((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *___op = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* arch_try_cmpxchg_local */
+
+#ifndef arch_try_cmpxchg64_local
+#define arch_try_cmpxchg64_local(_ptr, _oldp, _new) \
+({ \
+ typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \
+ ___r = arch_cmpxchg64_local((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *___op = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* arch_try_cmpxchg64_local */
+
#ifndef arch_atomic_read_acquire
static __always_inline int
arch_atomic_read_acquire(const atomic_t *v)
@@ -2456,4 +2478,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v)
#endif

#endif /* _LINUX_ATOMIC_FALLBACK_H */
-// b5e87bdd5ede61470c29f7a7e4de781af3770f09
+// 1f49bd4895a4b7a5383906649027205c52ec80ab
diff --git a/include/linux/atomic/atomic-instrumented.h b/include/linux/atomic/atomic-instrumented.h
index 7a139ec030b0..14a9212cc987 100644
--- a/include/linux/atomic/atomic-instrumented.h
+++ b/include/linux/atomic/atomic-instrumented.h
@@ -2066,6 +2066,24 @@ atomic_long_dec_if_positive(atomic_long_t *v)
arch_sync_cmpxchg(__ai_ptr, __VA_ARGS__); \
})

+#define try_cmpxchg_local(ptr, oldp, ...) \
+({ \
+ typeof(ptr) __ai_ptr = (ptr); \
+ typeof(oldp) __ai_oldp = (oldp); \
+ instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
+ instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \
+ arch_try_cmpxchg_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \
+})
+
+#define try_cmpxchg64_local(ptr, oldp, ...) \
+({ \
+ typeof(ptr) __ai_ptr = (ptr); \
+ typeof(oldp) __ai_oldp = (oldp); \
+ instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
+ instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \
+ arch_try_cmpxchg64_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \
+})
+
#define cmpxchg_double(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
@@ -2083,4 +2101,4 @@ atomic_long_dec_if_positive(atomic_long_t *v)
})

#endif /* _LINUX_ATOMIC_INSTRUMENTED_H */
-// 764f741eb77a7ad565dc8d99ce2837d5542e8aee
+// 456e206c7e4e681126c482e4edcc6f46921ac731
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 273a0fe7910a..111ab85ee97d 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle,

perf_output_get_handle(handle);

+ offset = local_read(&rb->head);
do {
tail = READ_ONCE(rb->user_page->data_tail);
- offset = head = local_read(&rb->head);
+ head = offset;
if (!rb->overwrite) {
if (unlikely(!ring_buffer_has_space(head, tail,
perf_data_size(rb),
@@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle,
head += size;
else
head -= size;
- } while (local_cmpxchg(&rb->head, offset, head) != offset);
+ } while (!local_try_cmpxchg(&rb->head, &offset, head));

if (backward) {
offset = head;
diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
index 3a07695e3c89..6e853f0dad8d 100755
--- a/scripts/atomic/gen-atomic-fallback.sh
+++ b/scripts/atomic/gen-atomic-fallback.sh
@@ -225,6 +225,10 @@ for cmpxchg in "cmpxchg" "cmpxchg64"; do
gen_try_cmpxchg_fallbacks "${cmpxchg}"
done

+for cmpxchg in "cmpxchg_local" "cmpxchg64_local"; do
+ gen_try_cmpxchg_fallback "${cmpxchg}" ""
+done
+
grep '^[a-z]' "$1" | while read name meta args; do
gen_proto "${meta}" "${name}" "atomic" "int" ${args}
done
diff --git a/scripts/atomic/gen-atomic-instrumented.sh b/scripts/atomic/gen-atomic-instrumented.sh
index 77c06526a574..c8165e9431bf 100755
--- a/scripts/atomic/gen-atomic-instrumented.sh
+++ b/scripts/atomic/gen-atomic-instrumented.sh
@@ -173,7 +173,7 @@ for xchg in "xchg" "cmpxchg" "cmpxchg64" "try_cmpxchg" "try_cmpxchg64"; do
done
done

-for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg"; do
+for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg" "try_cmpxchg_local" "try_cmpxchg64_local" ; do
gen_xchg "${xchg}" "" ""
printf "\n"
done

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In reply to: Mark Rutland: "Re: [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks"
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