| Message ID | 20210704071644.107397-6-leo.yan@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | perf: Refine barriers for AUX ring buffer | expand |
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@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, >
On Sat, Jul 10, 2021 at 03:34:24PM +0300, Adrian Hunter wrote: > 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. Okay, I admit the paragraph is not right, will drop it to avoid confusion. Thanks for review! Leo
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,
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(-)