From: Leo Yan <leo.yan@linaro.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
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>,
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>,
Adrian Hunter <adrian.hunter@intel.com>,
Andi Kleen <ak@linux.intel.com>,
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 v2 6/8] perf auxtrace: Drop legacy __sync functions
Date: Wed, 2 Jun 2021 18:30:05 +0800 [thread overview]
Message-ID: <20210602103007.184993-7-leo.yan@linaro.org> (raw)
In-Reply-To: <20210602103007.184993-1-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 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)
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,
--
2.25.1
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next prev parent reply other threads:[~2021-06-02 10:34 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-02 10:29 [PATCH v2 0/8] perf: Refine barriers for AUX ring buffer Leo Yan
2021-06-02 10:30 ` [PATCH v2 1/8] perf/ring_buffer: Add comment for barriers on " Leo Yan
2021-06-07 15:27 ` Peter Zijlstra
2021-06-02 10:30 ` [PATCH v2 2/8] coresight: tmc-etr: Add barrier after updating " Leo Yan
2021-06-02 10:30 ` [PATCH v2 3/8] coresight: tmc-etf: Add comment for store ordering Leo Yan
2021-06-02 10:30 ` [PATCH v2 4/8] perf/x86: Add barrier after updating bts Leo Yan
2021-06-07 15:29 ` Peter Zijlstra
2021-06-02 10:30 ` [PATCH v2 5/8] perf auxtrace: Change to use SMP memory barriers Leo Yan
2021-06-07 10:02 ` Adrian Hunter
2021-06-07 15:29 ` Peter Zijlstra
2021-06-08 16:45 ` Arnaldo Carvalho de Melo
2021-06-02 10:30 ` Leo Yan [this message]
2021-06-02 10:47 ` [PATCH v2 6/8] perf auxtrace: Drop legacy __sync functions Adrian Hunter
2021-06-02 11:16 ` Leo Yan
2021-06-02 11:21 ` Adrian Hunter
2021-06-02 13:01 ` Leo Yan
2021-06-02 10:30 ` [PATCH v2 7/8] perf auxtrace: Use WRITE_ONCE() for updating aux_tail Leo Yan
2021-06-07 10:03 ` Adrian Hunter
2021-06-07 15:31 ` Peter Zijlstra
2021-06-08 17:04 ` Arnaldo Carvalho de Melo
2021-06-09 0:21 ` Leo Yan
2021-06-02 10:30 ` [PATCH v2 8/8] perf record: Directly bail out for compat case Leo Yan
2021-06-02 11:18 ` Adrian Hunter
2021-06-02 12:38 ` Leo Yan
2021-06-07 10:23 ` Adrian Hunter
2021-06-07 15:09 ` Leo Yan
2021-06-09 8:23 ` Adrian Hunter
2021-06-09 8:57 ` Leo Yan
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