From: Stephen Hemminger <stephen@networkplumber.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: devel@linuxdriverproject.org, linux-kernel@vger.kernel.org,
"K. Y. Srinivasan" <kys@microsoft.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
John Stultz <john.stultz@linaro.org>,
Thomas Gleixner <tglx@linutronix.de>,
Alex Ng <alexng@microsoft.com>
Subject: Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts
Date: Wed, 4 Jan 2017 11:17:49 -0800 [thread overview]
Message-ID: <20170104111749.0e7d566e@xeon-e3> (raw)
In-Reply-To: <20170104172439.19683-5-vkuznets@redhat.com>
On Wed, 4 Jan 2017 18:24:39 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> If we happen to receive interrupts during hv_set_host_time() execution
> our adjustments may get inaccurate. Make the whole function atomic.
> Unfortunately, we can's call do_settimeofday64() with interrupts
> disabled as some cross-CPU work is being done but this call happens
> very rarely.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Ok, the race is between timer interrupts and calling do_adjtimex().
NTP has the same issue already.
The getnstimeofday64() (or ktime_get) return an atomic value.
If a clock tick interrupt happens during this code, then the value
is still correct just old.
If you want to avoid all races here, it looks like it would
be better to get timekeeper_lock and call __do_adjtimex. The existing
code in do_adjtimex() is expecting to be called from a system call
and changing it's assumptions is probably not a good idea.
Rather than calling system call from user space. Maybe better
to provide real kernel API in time subsystem for this use case.
What does KVM do?
next prev parent reply other threads:[~2017-01-04 19:18 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-04 17:24 [PATCH v2 0/4] hv_util: adjust system time smoothly Vitaly Kuznetsov
2017-01-04 17:24 ` [PATCH v2 1/4] timekeeping: export do_adjtimex() to modules Vitaly Kuznetsov
2017-01-07 1:06 ` John Stultz
2017-01-09 13:03 ` Vitaly Kuznetsov
2017-01-04 17:24 ` [PATCH v2 2/4] hv_util: switch to using timespec64 Vitaly Kuznetsov
2017-01-07 1:04 ` John Stultz
2017-01-04 17:24 ` [PATCH v2 3/4] hv_util: use do_adjtimex() to update system time Vitaly Kuznetsov
2017-01-04 19:09 ` Stephen Hemminger
2017-01-05 12:37 ` Vitaly Kuznetsov
2017-01-07 0:56 ` John Stultz
2017-01-04 17:24 ` [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts Vitaly Kuznetsov
2017-01-04 19:17 ` Stephen Hemminger [this message]
2017-01-05 12:35 ` Vitaly Kuznetsov
2017-01-05 17:39 ` Stephen Hemminger
2017-01-07 1:02 ` John Stultz
2017-01-09 13:05 ` Vitaly Kuznetsov
2017-01-09 21:27 ` [PATCH v2 0/4] hv_util: adjust system time smoothly Thomas Gleixner
2017-01-10 14:30 ` Vitaly Kuznetsov
2017-01-10 14:58 ` Thomas Gleixner
2017-01-13 13:05 ` [PATCH RFC] hv_utils: implement Hyper-V PTP source Vitaly Kuznetsov
2017-01-13 14:50 ` Richard Cochran
2017-01-13 15:38 ` Vitaly Kuznetsov
2017-01-13 15:21 ` Olaf Hering
2017-01-13 15:37 ` Vitaly Kuznetsov
2017-01-16 19:29 ` Thomas Gleixner
2017-01-17 9:53 ` Vitaly Kuznetsov
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