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Peter Anvin" , Mathieu Poirier , Suzuki K Poulose , Mike Leach , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v4 03/11] coresight: tmc-etf: Add comment for store ordering Message-ID: <20210713154935.GF748506@leoy-ThinkPad-X240s> References: <20210711104105.505728-1-leo.yan@linaro.org> <20210711104105.505728-4-leo.yan@linaro.org> <20210713125606.GB4170@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210713125606.GB4170@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 13, 2021 at 02:56:06PM +0200, Peter Zijlstra wrote: > On Sun, Jul 11, 2021 at 06:40:57PM +0800, Leo Yan wrote: > > AUX ring buffer is required to separate the data store and aux_head > > store, since the function CS_LOCK() has contained memory barrier mb(), > > mb() is a more conservative barrier than smp_wmb() on Arm32/Arm64, thus > > it's needless to add any explicit barrier anymore. > > > > Add comment to make clear for the barrier usage for ETF. > > > > Signed-off-by: Leo Yan > > --- > > drivers/hwtracing/coresight/coresight-tmc-etf.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c b/drivers/hwtracing/coresight/coresight-tmc-etf.c > > index 45b85edfc690..9a42ee689921 100644 > > --- a/drivers/hwtracing/coresight/coresight-tmc-etf.c > > +++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c > > @@ -553,6 +553,12 @@ static unsigned long tmc_update_etf_buffer(struct coresight_device *csdev, > > if (buf->snapshot) > > handle->head += to_read; > > > > + /* > > + * AUX ring buffer requires to use memory barrier to separate the trace > > + * data store and aux_head store, because CS_LOCK() contains mb() which > > + * gives more heavy barrier than smp_wmb(), it's not necessary to > > + * explicitly invoke any barrier. > > + */ > > CS_LOCK(drvdata->base); > > 'more heavy' is not a correctness argument :-) > > The argument to make here is that CS_LOCK() ensures completion / > visibility of the hardware buffer. Will correct for this, thanks for reminding!