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From: Leo Yan <leo.yan@linaro.org>
To: Adrian Hunter <adrian.hunter@intel.com>
Cc: 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>,
	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
Subject: Re: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions
Date: Sun, 11 Jul 2021 15:41:16 +0800	[thread overview]
Message-ID: <20210711074116.GC273828@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <26efa59c-4bdd-b93a-7ab8-4bb270aaee93@intel.com>

On Sat, Jul 10, 2021 at 03:34:24PM +0300, Adrian Hunter wrote:
> On 4/07/21 10:16 am, Leo Yan wrote:
> > 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'.
> 
> That is not exactly true. The definition of __sync_*_compare_and_swap is
> "if the current value of *ptr is oldval, then write newval into *pt"
> so replacing zero with zero won't make any difference, but it will return
> the old value in any case.  Probably better to leave out that paragraph.

Okay, I admit the paragraph is not right, will drop it to avoid
confusion.  Thanks for review!

Leo

  reply	other threads:[~2021-07-11  7:41 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 ` [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions Leo Yan
2021-07-10 12:34   ` Adrian Hunter
2021-07-11  7:41     ` Leo Yan [this message]
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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