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_005fsync-Builtins
>
> Suggested-by: Peter Zijlstra <[email protected]>
> Signed-off-by: Leo Yan <[email protected]>
> ---
> 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,
>
Hi Adrian,
On Wed, Jun 02, 2021 at 01:47:42PM +0300, Adrian Hunter wrote:
[...]
> > @@ -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.
Yes, I think there have two files should be cleaned:
Makefile.config
util/auxtrace.c
If still miss anything, please let me know (I remembered there have a
test case for __sync_xxx_compare_and_swap, but I cannot find it now,
so this is why I am concern if I miss anything not).
Thanks for review,
Leo
On 2/06/21 2:16 pm, Leo Yan wrote:
> Hi Adrian,
>
> On Wed, Jun 02, 2021 at 01:47:42PM +0300, Adrian Hunter wrote:
>
> [...]
>
>>> @@ -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.
>
> Yes, I think there have two files should be cleaned:
>
> Makefile.config
> util/auxtrace.c
>
> If still miss anything, please let me know (I remembered there have a
> test case for __sync_xxx_compare_and_swap, but I cannot find it now,
> so this is why I am concern if I miss anything not).
tools/build/feature/test-sync-compare-and-swap.c
On Wed, Jun 02, 2021 at 02:21:05PM +0300, Adrian Hunter wrote:
> On 2/06/21 2:16 pm, Leo Yan wrote:
> > Hi Adrian,
> >
> > On Wed, Jun 02, 2021 at 01:47:42PM +0300, Adrian Hunter wrote:
> >
> > [...]
> >
> >>> @@ -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.
> >
> > Yes, I think there have two files should be cleaned:
> >
> > Makefile.config
> > util/auxtrace.c
> >
> > If still miss anything, please let me know (I remembered there have a
> > test case for __sync_xxx_compare_and_swap, but I cannot find it now,
> > so this is why I am concern if I miss anything not).
>
> tools/build/feature/test-sync-compare-and-swap.c
Thanks a lot!