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Peter Anvin" , Mathieu Poirier , Suzuki K Poulose , Mike Leach , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org Cc: Leo Yan Subject: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions Date: Sun, 4 Jul 2021 15:16:39 +0800 Message-Id: <20210704071644.107397-6-leo.yan@linaro.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20210704071644.107397-1-leo.yan@linaro.org> References: <20210704071644.107397-1-leo.yan@linaro.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Signed-off-by: Leo Yan --- 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