Re: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions

[Adrian Hunter]

On 4/07/21 10:16 am, 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'.

That is not exactly true. The definition of __sync_*_compare_and_swap is
"if the current value of *ptr is oldval, then write newval into *pt"
so replacing zero with zero won't make any difference, but it will return
the old value in any case.  Probably better to leave out that paragraph.

> 
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>  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,
>