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Darwish" Cc: Thomas Gleixner , Peter Zijlstra , Will Deacon , John Ogness , "Sebastian A. Siewior" , LKML Subject: Re: [PATCH] time/sched_clock: Use raw_read_seqcount_latch() Message-ID: <20200715152926.GA16686@leoy-ThinkPad-X240s> References: <20200715095220.GH10769@hirez.programming.kicks-ass.net> <20200715115901.515956-1-a.darwish@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200715115901.515956-1-a.darwish@linutronix.de> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, Ahmed, On Wed, Jul 15, 2020 at 01:59:01PM +0200, Ahmed S. Darwish wrote: > sched_clock uses seqcount_t latching to switch between two storage > places protected by the sequence counter. This allows it to have > interruptible, NMI-safe, seqcount_t write side critical sections. > > Since 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"), > raw_read_seqcount_latch() became the standardized way for seqcount_t > latch read paths. Due to the dependent load, it also has one read > memory barrier less than the currently used raw_read_seqcount() API. > > Use raw_read_seqcount_latch() for the seqcount_t latch read path. > > Link: https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net > Link: https://lkml.kernel.org/r/20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de > References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI") > Signed-off-by: Ahmed S. Darwish > --- > kernel/time/sched_clock.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c > index fa3f800d7d76..ea007928d681 100644 > --- a/kernel/time/sched_clock.c > +++ b/kernel/time/sched_clock.c > @@ -100,7 +100,7 @@ unsigned long long notrace sched_clock(void) > struct clock_read_data *rd; > > do { > - seq = raw_read_seqcount(&cd.seq); > + seq = raw_read_seqcount_latch(&cd.seq); Understand this is doing the same thing with __ktime_get_fast_ns() and I saw Peter acked to make change for this. Just want to confirm, since this patch introduces conflict with the patch set "arm64: perf: Proper cap_user_time* support" [1], I should rebase the patch set on top of this patch, right? Thanks, Leo [1] https://patchwork.kernel.org/cover/11664031/ > rd = cd.read_data + (seq & 1); > > cyc = (rd->read_sched_clock() - rd->epoch_cyc) & > -- > 2.20.1