linux-kernel - [PATCH v2] clockevents: Retry programming min delta up to 10 times

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 19 Oct 2017 15:17:23 +0100
From:   Matt Redfearn <matt.redfearn@...s.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
CC:     James Hogan <james.hogan@...s.com>, <linux-mips@...ux-mips.org>,
        "James Hogan" <jhogan@...nel.org>,
        Matt Redfearn <matt.redfearn@...s.com>,
        "Martin Schwidefsky" <schwidefsky@...ibm.com>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] clockevents: Retry programming min delta up to 10 times

From: James Hogan <jhogan@...nel.org>

When CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST=n, the call path
hrtimer_reprogram -> clockevents_program_event ->
clockevents_program_min_delta will not retry if the clock event driver
returns -ETIME.

If the driver could not satisfy the program_min_delta for any reason,
the lack of a retry means the CPU may not receive a tick interrupt,
potentially until the counter does a full period. This leads to
rcu_sched timeout messages as the stalled CPU is detected by other CPUs,
and other issues if the CPU is holding locks or other resources at the
point at which it stalls.

There have been a couple of observed mechanisms through which a clock
event driver could not satisfy the requested min_delta and return
-ETIME.

With the MIPS GIC driver, the shared execution resource within MT cores
means inconventient latency due to execution of instructions from other
hardware threads in the core, within gic_next_event, can result in an
event being set in the past.

Additionally under virtualisation it is possible to get unexpected
latency during a clockevent device's set_next_event() callback which can
make it return -ETIME even for a delta based on min_delta_ns.

It isn't appropriate to use MIN_ADJUST in the virtualisation case as
occasional hypervisor induced high latency will cause min_delta_ns to
quickly increase to the maximum.

Instead, borrow the retry pattern from the MIN_ADJUST case, but without
making adjustments. We retry up to 10 times, each time increasing the
attempted delta by min_delta, before giving up.

Signed-off-by: James Hogan <jhogan@...nel.org>
Signed-off-by: Matt Redfearn <matt.redfearn@...s.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>

---

Changes in v2:
Restructure for loop and retry with increasing multiples of min_delta.

 kernel/time/clockevents.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 4237e0744e26..84c2e3b57428 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -281,16 +281,22 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
 {
 	unsigned long long clc;
 	int64_t delta;
+	int i;

-	delta = dev->min_delta_ns;
-	dev->next_event = ktime_add_ns(ktime_get(), delta);
+	delta = 0;
+	for (i = 0; i < 10; i++) {
+		delta += dev->min_delta_ns;
+		dev->next_event = ktime_add_ns(ktime_get(), delta);

-	if (clockevent_state_shutdown(dev))
-		return 0;
+		if (clockevent_state_shutdown(dev))
+			return 0;

-	dev->retries++;
-	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-	return dev->set_next_event((unsigned long) clc, dev);
+		dev->retries++;
+		clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
+		if (dev->set_next_event((unsigned long) clc, dev) == 0)
+			return 0;
+	}
+	return -ETIME;
 }

 #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
-- 
2.7.4