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Subject[PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions
Date
The main purpose for using __sync built-in functions is to support
compat mode for 32-bit perf with 64-bit kernel. But using these
built-in functions might cause couple potential issues.

Firstly, __sync functions originally support Intel Itanium processoer [1]
but it cannot promise to support all 32-bit archs. Now these
functions have become the legacy functions.

As Peter also pointed out the logic issue in the function
auxtrace_mmap__write_tail(), it does a cmpxchg with 0 values to load
old_tail, and then executes a further cmpxchg with old_tail to write
the new tail. If consider the aux_tail might be assigned to '0' in the
middle of loops, this can introduce mess for AUX buffer if the kernel
fetches the temporary value '0'.

Considering __sync functions cannot really fix the 64-bit value
atomicity on 32-bit archs, thus this patch drops __sync functions.

Credits to Peter for detailed analysis.

[1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
---
tools/perf/util/auxtrace.h | 19 -------------------
1 file changed, 19 deletions(-)

diff --git a/tools/perf/util/auxtrace.h b/tools/perf/util/auxtrace.h
index cc1c1b9cec9c..f489ca159997 100644
--- a/tools/perf/util/auxtrace.h
+++ b/tools/perf/util/auxtrace.h
@@ -440,12 +440,6 @@ struct auxtrace_cache;

#ifdef HAVE_AUXTRACE_SUPPORT

-/*
- * In snapshot mode the mmapped page is read-only which makes using
- * __sync_val_compare_and_swap() problematic. However, snapshot mode expects
- * the buffer is not updated while the snapshot is made (e.g. Intel PT disables
- * the event) so there is not a race anyway.
- */
static inline u64 auxtrace_mmap__read_snapshot_head(struct auxtrace_mmap *mm)
{
struct perf_event_mmap_page *pc = mm->userpg;
@@ -459,11 +453,7 @@ static inline u64 auxtrace_mmap__read_snapshot_head(struct auxtrace_mmap *mm)
static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
{
struct perf_event_mmap_page *pc = mm->userpg;
-#if BITS_PER_LONG == 64 || !defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT)
u64 head = READ_ONCE(pc->aux_head);
-#else
- u64 head = __sync_val_compare_and_swap(&pc->aux_head, 0, 0);
-#endif

/* Ensure all reads are done after we read the head */
smp_rmb();
@@ -473,19 +463,10 @@ static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
static inline void auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
{
struct perf_event_mmap_page *pc = mm->userpg;
-#if BITS_PER_LONG != 64 && defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT)
- u64 old_tail;
-#endif

/* Ensure all reads are done before we write the tail out */
smp_mb();
-#if BITS_PER_LONG == 64 || !defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT)
pc->aux_tail = tail;
-#else
- do {
- old_tail = __sync_val_compare_and_swap(&pc->aux_tail, 0, 0);
- } while (!__sync_bool_compare_and_swap(&pc->aux_tail, old_tail, tail));
-#endif
}

int auxtrace_mmap__mmap(struct auxtrace_mmap *mm,
--
2.25.1
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