On 2/06/21 1:30 pm, Leo Yan wrote:
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_005...
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 42b7ef811bde..e625bc76cdde 100644 --- a/tools/perf/util/auxtrace.h +++ b/tools/perf/util/auxtrace.h @@ -432,12 +432,6 @@ 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; @@ -451,11 +445,7 @@ 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)
The test and setup for HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT is not needed anymore either.
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(); @@ -465,19 +455,10 @@ 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,