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
next prev 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210704071644.107397-6-leo.yan@linaro.org \
--to=leo.yan@linaro.org \
--cc=acme@kernel.org \
--cc=adrian.hunter@intel.com \
--cc=alexander.shishkin@linux.intel.com \
--cc=bp@alien8.de \
--cc=coresight@lists.linaro.org \
--cc=hpa@zytor.com \
--cc=jolsa@redhat.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-perf-users@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mathieu.poirier@linaro.org \
--cc=mike.leach@linaro.org \
--cc=mingo@redhat.com \
--cc=namhyung@kernel.org \
--cc=peterz@infradead.org \
--cc=suzuki.poulose@arm.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).