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From: Leo Yan <leo.yan@linaro.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Cc: Leo Yan <leo.yan@linaro.org>
Subject: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions
Date: Sun,  4 Jul 2021 15:16:39 +0800	[thread overview]
Message-ID: <20210704071644.107397-6-leo.yan@linaro.org> (raw)
In-Reply-To: <20210704071644.107397-1-leo.yan@linaro.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 <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,
-- 
2.25.1


  parent reply	other threads:[~2021-07-04  7:17 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-04  7:16 [PATCH v3 00/10] perf: Refine barriers for AUX ring buffer Leo Yan
2021-07-04  7:16 ` [PATCH v3 01/10] perf/ring_buffer: Add comment for barriers on " Leo Yan
2021-07-04  7:16 ` [PATCH v3 02/10] coresight: tmc-etr: Add barrier after updating " Leo Yan
2021-07-04  7:16 ` [PATCH v3 03/10] coresight: tmc-etf: Add comment for store ordering Leo Yan
2021-07-04  7:16 ` [PATCH v3 04/10] perf/x86: Add barrier after updating bts Leo Yan
2021-07-04  7:16 ` Leo Yan [this message]
2021-07-10 12:34   ` [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions Adrian Hunter
2021-07-11  7:41     ` Leo Yan
2021-07-04  7:16 ` [PATCH v3 06/10] perf auxtrace: Remove auxtrace_mmap__read_snapshot_head() Leo Yan
2021-07-04  7:16 ` [PATCH v3 07/10] perf: Cleanup for HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT Leo Yan
2021-07-10 12:36   ` Adrian Hunter
2021-07-11  8:08     ` Leo Yan
2021-07-04  7:16 ` [PATCH v3 08/10] perf auxtrace: Use WRITE_ONCE() for updating aux_tail Leo Yan
2021-07-04  7:16 ` [PATCH v3 09/10] perf env: Set kernel bit mode Leo Yan
2021-07-04  7:16 ` [PATCH v3 10/10] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail} Leo Yan

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