[PATCH v5 6/9] perf auxtrace: Drop legacy __sync functions

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>,
	"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>,
	"Michael Petlan" <mpetlan@redhat.com>,
	"Frank Ch. Eigler" <fche@redhat.com>,
	"Song Liu" <songliubraving@fb.com>,
	x86@kernel.org, "Daniel Díaz" <daniel.diaz@linaro.org>,
	"Andrii Nakryiko" <andriin@fb.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Sedat Dilek" <sedat.dilek@gmail.com>,
	"Andi Kleen" <ak@linux.intel.com>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org
Cc: Leo Yan <leo.yan@linaro.org>
Subject: [PATCH v5 6/9] perf auxtrace: Drop legacy __sync functions
Date: Mon,  9 Aug 2021 19:14:04 +0800	[thread overview]
Message-ID: <20210809111407.596077-7-leo.yan@linaro.org> (raw)
In-Reply-To: <20210809111407.596077-1-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 potential issues.

__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.

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 79227b8864cd..4f9176368134 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)
 	WRITE_ONCE(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

