From: Arnd Bergmann <arnd@arndb.de>
To: Pingbo Wen <pingbo.wen@linaro.org>
Cc: y2038@lists.linaro.org, dmitry.torokhov@gmail.com,
linux-kernel@vger.kernel.org, linux-input@vger.kernel.org
Subject: Re: [Y2038] [PATCH] hil_mlc: convert timeval to timespec64
Date: Fri, 23 Oct 2015 11:45:59 +0200 [thread overview]
Message-ID: <5201254.PnVcGHTDmH@wuerfel> (raw)
In-Reply-To: <5629FA06.2060608@linaro.org>
On Friday 23 October 2015 17:12:38 Pingbo Wen wrote:
> On Monday, October 19, 2015 04:58 PM, Arnd Bergmann wrote:
> >> - do_gettimeofday(&tv);
> >> - tv.tv_usec += USEC_PER_SEC * (tv.tv_sec - mlc->instart.tv_sec);
> >> - tv.tv_usec -= mlc->instart.tv_usec;
> >> - if (tv.tv_usec >= mlc->intimeout) goto sched;
> >> - tv.tv_usec = (mlc->intimeout - tv.tv_usec) * HZ / USEC_PER_SEC;
> >> - if (!tv.tv_usec) goto sched;
> >> - mod_timer(&hil_mlcs_kicker, jiffies + tv.tv_usec);
> >> + ktime_get_ts64(&ts64);
> >> + ts64.tv_nsec += NSEC_PER_SEC *
> >> + (ts64.tv_sec - mlc->instart.tv_sec);
> >> + ts64.tv_nsec -= mlc->instart.tv_nsec;
> >
> > tv_nsec will overflow here for any timeout over 4.3 seconds, where it
> > used to overflow after 4294 seconds. This is almost certainly a bug.
> >
> > You could work around that by using ktime_get_ns() to get a nanosecond
> > value right away, but a 64-bit number is more expensive to convert to
> > jiffies.
>
> You are right, I didn't notice that tv_nsec is a 32bit variable. Maybe
> we should use ktime_t here, so that handling sec and nsec separately is
> needless.
>
> Using jiffies here will need to take more codes to handle jiffies overflow
> carefully. I think coverting 64bit number to jiffies is the price we must
> take, if we use 64bit version here.
Handling the jiffies overflow is trivially done through the time_before()
and time_after() helpers, like
start = jiffies;
...
now = jiffies;
timeout = start + HZ * timeout_usec / USEC_PER_SEC;
if (time_after(now, start + timeout_jiffies)
timeout();
else
mod_timer(timer, start + timeout_jiffies);
The time_after function works because unsigned overflow is well-defined
in C (unlike signed overflow).
As a side-note, please take the time to delete any lines from the original
mail that you are not referencing when you reply.
Arnd
WARNING: multiple messages have this Message-ID (diff)
From: Arnd Bergmann <arnd@arndb.de>
To: Pingbo Wen <pingbo.wen@linaro.org>
Cc: y2038@lists.linaro.org, dmitry.torokhov@gmail.com,
linux-kernel@vger.kernel.org, linux-input@vger.kernel.org
Subject: Re: [PATCH] hil_mlc: convert timeval to timespec64
Date: Fri, 23 Oct 2015 11:45:59 +0200 [thread overview]
Message-ID: <5201254.PnVcGHTDmH@wuerfel> (raw)
In-Reply-To: <5629FA06.2060608@linaro.org>
On Friday 23 October 2015 17:12:38 Pingbo Wen wrote:
> On Monday, October 19, 2015 04:58 PM, Arnd Bergmann wrote:
> >> - do_gettimeofday(&tv);
> >> - tv.tv_usec += USEC_PER_SEC * (tv.tv_sec - mlc->instart.tv_sec);
> >> - tv.tv_usec -= mlc->instart.tv_usec;
> >> - if (tv.tv_usec >= mlc->intimeout) goto sched;
> >> - tv.tv_usec = (mlc->intimeout - tv.tv_usec) * HZ / USEC_PER_SEC;
> >> - if (!tv.tv_usec) goto sched;
> >> - mod_timer(&hil_mlcs_kicker, jiffies + tv.tv_usec);
> >> + ktime_get_ts64(&ts64);
> >> + ts64.tv_nsec += NSEC_PER_SEC *
> >> + (ts64.tv_sec - mlc->instart.tv_sec);
> >> + ts64.tv_nsec -= mlc->instart.tv_nsec;
> >
> > tv_nsec will overflow here for any timeout over 4.3 seconds, where it
> > used to overflow after 4294 seconds. This is almost certainly a bug.
> >
> > You could work around that by using ktime_get_ns() to get a nanosecond
> > value right away, but a 64-bit number is more expensive to convert to
> > jiffies.
>
> You are right, I didn't notice that tv_nsec is a 32bit variable. Maybe
> we should use ktime_t here, so that handling sec and nsec separately is
> needless.
>
> Using jiffies here will need to take more codes to handle jiffies overflow
> carefully. I think coverting 64bit number to jiffies is the price we must
> take, if we use 64bit version here.
Handling the jiffies overflow is trivially done through the time_before()
and time_after() helpers, like
start = jiffies;
...
now = jiffies;
timeout = start + HZ * timeout_usec / USEC_PER_SEC;
if (time_after(now, start + timeout_jiffies)
timeout();
else
mod_timer(timer, start + timeout_jiffies);
The time_after function works because unsigned overflow is well-defined
in C (unlike signed overflow).
As a side-note, please take the time to delete any lines from the original
mail that you are not referencing when you reply.
Arnd
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next prev parent reply other threads:[~2015-10-23 9:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-18 9:45 [PATCH] hil_mlc: convert timeval to timespec64 WEN Pingbo
2015-10-19 8:58 ` [Y2038] " Arnd Bergmann
2015-10-23 9:12 ` Pingbo Wen
2015-10-23 9:45 ` Arnd Bergmann [this message]
2015-10-23 9:45 ` Arnd Bergmann
2015-10-23 12:34 ` [Y2038] " Pingbo Wen
2015-10-23 12:34 ` Pingbo Wen
2015-10-24 4:00 ` [PATCH V3 1/2] hil_mlc: convert timeval to time64_t WEN Pingbo
2015-10-24 4:00 ` WEN Pingbo
2015-10-24 4:00 ` [PATCH V3 2/2] hil_mlc: convert timeval to jiffies WEN Pingbo
2015-10-24 4:00 ` WEN Pingbo
2015-10-23 9:24 ` [PATCH V2] hil_mlc: convert timeval to ktime_t WEN Pingbo
2015-10-23 9:24 ` WEN Pingbo
2015-10-23 9:55 ` [Y2038] " Arnd Bergmann
2015-10-23 12:32 ` Pingbo Wen
2015-10-23 13:51 ` Arnd Bergmann
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