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From: John Stultz <john.stultz@linaro.org>
To: lkml <linux-kernel@vger.kernel.org>
Cc: Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Miroslav Lichvar <mlichvar@redhat.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Prarit Bhargava <prarit@redhat.com>,
	Stephen Boyd <stephen.boyd@linaro.org>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Daniel Mentz <danielmentz@google.com>,
	"stable #4 . 8+" <stable@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>
Subject: [PATCH 3/3 v3] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW
Date: Thu,  8 Jun 2017 16:44:22 -0700
Message-ID: <1496965462-20003-4-git-send-email-john.stultz@linaro.org> (raw)
In-Reply-To: <1496965462-20003-1-git-send-email-john.stultz@linaro.org>

From: Will Deacon <will.deacon@arm.com>

Recently vDSO support for CLOCK_MONOTONIC_RAW was added in
49eea433b326 ("arm64: Add support for CLOCK_MONOTONIC_RAW in
clock_gettime() vDSO"). Noticing that the core timekeeping code
never set tkr_raw.xtime_nsec, the vDSO implementation didn't
bother exposing it via the data page and instead took the
unshifted tk->raw_time.tv_nsec value which was then immediately
shifted left in the vDSO code.

Unfortunately, by accellerating the MONOTONIC_RAW clockid, it
uncovered potential 1ns time inconsistencies caused by the
timekeeping core not handing sub-ns resolution.

Now that the core code has been fixed and is actually setting
tkr_raw.xtime_nsec, we need to take that into account in the
vDSO by adding it to the shifted raw_time value, in order to
fix the user-visible inconsistency. Rather than do that at each
use (and expand the data page in the process), instead perform
the shift/addition operation when populating the data page and
remove the shift from the vDSO code entirely.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Daniel Mentz <danielmentz@google.com>
Cc: stable <stable@vger.kernel.org> #4.8+
Reported-by: John Stultz <john.stultz@linaro.org>
Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>
Tested-by: Daniel Mentz <danielmentz@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[jstultz: minor whitespace tweak, tried to improve commit
 message to make it more clear this fixes a regression]
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
v2: Tweak commit message to address Ingo's feedback
---
 arch/arm64/kernel/vdso.c              | 5 +++--
 arch/arm64/kernel/vdso/gettimeofday.S | 1 -
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 41b6e31..d0cb007 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -221,10 +221,11 @@ void update_vsyscall(struct timekeeper *tk)
 		/* tkr_mono.cycle_last == tkr_raw.cycle_last */
 		vdso_data->cs_cycle_last	= tk->tkr_mono.cycle_last;
 		vdso_data->raw_time_sec		= tk->raw_time.tv_sec;
-		vdso_data->raw_time_nsec	= tk->raw_time.tv_nsec;
+		vdso_data->raw_time_nsec	= (tk->raw_time.tv_nsec <<
+						   tk->tkr_raw.shift) +
+						  tk->tkr_raw.xtime_nsec;
 		vdso_data->xtime_clock_sec	= tk->xtime_sec;
 		vdso_data->xtime_clock_nsec	= tk->tkr_mono.xtime_nsec;
-		/* tkr_raw.xtime_nsec == 0 */
 		vdso_data->cs_mono_mult		= tk->tkr_mono.mult;
 		vdso_data->cs_raw_mult		= tk->tkr_raw.mult;
 		/* tkr_mono.shift == tkr_raw.shift */
diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
index e00b467..76320e9 100644
--- a/arch/arm64/kernel/vdso/gettimeofday.S
+++ b/arch/arm64/kernel/vdso/gettimeofday.S
@@ -256,7 +256,6 @@ monotonic_raw:
 	seqcnt_check fail=monotonic_raw
 
 	/* All computations are done with left-shifted nsecs. */
-	lsl	x14, x14, x12
 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12
 
-- 
2.7.4

  parent reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-08 23:44 [PATCH 0/3 v3] Fixes for two recently found timekeeping bugs John Stultz
2017-06-08 23:44 ` [PATCH 1/3 v3] time: Fix clock->read(clock) race around clocksource changes John Stultz
2017-06-20  8:46   ` [tip:timers/urgent] " tip-bot for John Stultz
2017-06-08 23:44 ` [PATCH 2/3 v3] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting John Stultz
2017-06-13 10:30   ` Miroslav Lichvar
2017-06-20  8:46   ` [tip:timers/urgent] " tip-bot for John Stultz
2017-06-08 23:44 ` John Stultz [this message]
2017-06-20  8:47   ` [tip:timers/urgent] arm64/vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW tip-bot for Will Deacon

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