* Re: [PATCH] ktime: Fix ktime_divns to do signed division
2015-05-01 22:41 [PATCH] ktime: Fix ktime_divns to do signed division John Stultz
@ 2015-05-01 22:43 ` John Stultz
2015-05-01 23:54 ` Nicolas Pitre
` (2 subsequent siblings)
3 siblings, 0 replies; 6+ messages in thread
From: John Stultz @ 2015-05-01 22:43 UTC (permalink / raw)
To: lkml
Cc: John Stultz, Nicolas Pitre, Thomas Gleixner, Josh Boyer,
One Thousand Gnomes
Bah. I forgot to add [RFC] to the subject. This patch isn't yet ready
for submission, I just wanted to get some initial feedback on it.
thanks
-john
On Fri, May 1, 2015 at 3:41 PM, John Stultz <john.stultz@linaro.org> wrote:
> It was noted that the 32bit implementation of ktime_divns
> was doing unsgined division adn didn't properly handle
> negative values.
>
> This patch fixes the problem by checking and preserving
> the sign bit, and then reapplying it if appropriate
> after the division.
>
> Unfortunately there is some duplication since we have
> the optimized version for constant 32bit divider. I
> was considering reworkign the __ktime_divns helper
> to simplify the sign-handling logic, but then it
> would likely just be a s64/s64 divide, and probably
> should be more generic.
>
> Thoughts?
>
> Nicolas also notes that the ktime_divns() function
> breaks if someone passes in a negative divisor as
> well. This patch doesn't yet address that issue.
>
> Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Josh Boyer <jwboyer@redhat.com>
> Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
> Reported-by: Trevor Cordes <trevor@tecnopolis.ca>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
> include/linux/ktime.h | 12 ++++++++++--
> kernel/time/hrtimer.c | 11 +++++++++--
> 2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..d947263 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,12 +166,20 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
> }
>
> #if BITS_PER_LONG < 64
> -extern u64 __ktime_divns(const ktime_t kt, s64 div);
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
> static inline u64 ktime_divns(const ktime_t kt, s64 div)
> {
> if (__builtin_constant_p(div) && !(div >> 32)) {
> - u64 ns = kt.tv64;
> + s64 ns = kt.tv64;
> + int neg = 0;
> +
> + if (ns < 0) {
> + neg = 1;
> + ns = -ns;
> + }
> do_div(ns, div);
> + if (neg)
> + ns = -ns;
> return ns;
> } else {
> return __ktime_divns(kt, div);
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..4c1b294 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,17 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
> /*
> * Divide a ktime value by a nanosecond value
> */
> -u64 __ktime_divns(const ktime_t kt, s64 div)
> +s64 __ktime_divns(const ktime_t kt, s64 div)
> {
> - u64 dclc;
> + s64 dclc;
> int sft = 0;
> + int neg = 0;
>
> dclc = ktime_to_ns(kt);
> + if (dclc < 0) {
> + neg = 1;
> + dclc = -dclc;
> + }
> /* Make sure the divisor is less than 2^32: */
> while (div >> 32) {
> sft++;
> @@ -279,6 +284,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
> }
> dclc >>= sft;
> do_div(dclc, (unsigned long) div);
> + if (neg)
> + dclc = -dclc;
>
> return dclc;
> }
> --
> 1.9.1
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] ktime: Fix ktime_divns to do signed division
2015-05-01 22:41 [PATCH] ktime: Fix ktime_divns to do signed division John Stultz
2015-05-01 22:43 ` John Stultz
@ 2015-05-01 23:54 ` Nicolas Pitre
2015-05-02 0:00 ` John Stultz
2015-05-02 0:14 ` Nicolas Pitre
2015-05-02 8:31 ` Trevor Cordes
3 siblings, 1 reply; 6+ messages in thread
From: Nicolas Pitre @ 2015-05-01 23:54 UTC (permalink / raw)
To: John Stultz
Cc: linux-kernel, Thomas Gleixner, Josh Boyer, One Thousand Gnomes
On Fri, 1 May 2015, John Stultz wrote:
> It was noted that the 32bit implementation of ktime_divns
> was doing unsgined division adn didn't properly handle
> negative values.
>
> This patch fixes the problem by checking and preserving
> the sign bit, and then reapplying it if appropriate
> after the division.
>
> Unfortunately there is some duplication since we have
> the optimized version for constant 32bit divider. I
> was considering reworkign the __ktime_divns helper
> to simplify the sign-handling logic, but then it
> would likely just be a s64/s64 divide, and probably
> should be more generic.
>
> Thoughts?
Wouldn't it be better to simply forbid negative time altogether? Given
it's been broken for quite a while, there must not be that many
instances of such usage and fixing them would avoid the useless sign
handling overhead to 99.9% of the cases.
> Nicolas also notes that the ktime_divns() function
> breaks if someone passes in a negative divisor as
> well. This patch doesn't yet address that issue.
GRanted, a negative divisor here would be even weirder and should
definitely be rejected. Maybe the infinite loop is a good thing in that
case, probably better than producing wrong numbers.
> Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Josh Boyer <jwboyer@redhat.com>
> Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
> Reported-by: Trevor Cordes <trevor@tecnopolis.ca>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
> include/linux/ktime.h | 12 ++++++++++--
> kernel/time/hrtimer.c | 11 +++++++++--
> 2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..d947263 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,12 +166,20 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
> }
>
> #if BITS_PER_LONG < 64
> -extern u64 __ktime_divns(const ktime_t kt, s64 div);
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
> static inline u64 ktime_divns(const ktime_t kt, s64 div)
> {
> if (__builtin_constant_p(div) && !(div >> 32)) {
> - u64 ns = kt.tv64;
> + s64 ns = kt.tv64;
> + int neg = 0;
> +
> + if (ns < 0) {
> + neg = 1;
> + ns = -ns;
> + }
> do_div(ns, div);
> + if (neg)
> + ns = -ns;
> return ns;
> } else {
> return __ktime_divns(kt, div);
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..4c1b294 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,17 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
> /*
> * Divide a ktime value by a nanosecond value
> */
> -u64 __ktime_divns(const ktime_t kt, s64 div)
> +s64 __ktime_divns(const ktime_t kt, s64 div)
> {
> - u64 dclc;
> + s64 dclc;
> int sft = 0;
> + int neg = 0;
>
> dclc = ktime_to_ns(kt);
> + if (dclc < 0) {
> + neg = 1;
> + dclc = -dclc;
> + }
> /* Make sure the divisor is less than 2^32: */
> while (div >> 32) {
> sft++;
> @@ -279,6 +284,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
> }
> dclc >>= sft;
> do_div(dclc, (unsigned long) div);
> + if (neg)
> + dclc = -dclc;
>
> return dclc;
> }
> --
> 1.9.1
>
> --
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>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] ktime: Fix ktime_divns to do signed division
2015-05-01 23:54 ` Nicolas Pitre
@ 2015-05-02 0:00 ` John Stultz
0 siblings, 0 replies; 6+ messages in thread
From: John Stultz @ 2015-05-02 0:00 UTC (permalink / raw)
To: Nicolas Pitre; +Cc: lkml, Thomas Gleixner, Josh Boyer, One Thousand Gnomes
On Fri, May 1, 2015 at 4:54 PM, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> On Fri, 1 May 2015, John Stultz wrote:
>
>> It was noted that the 32bit implementation of ktime_divns
>> was doing unsgined division adn didn't properly handle
>> negative values.
>>
>> This patch fixes the problem by checking and preserving
>> the sign bit, and then reapplying it if appropriate
>> after the division.
>>
>> Unfortunately there is some duplication since we have
>> the optimized version for constant 32bit divider. I
>> was considering reworkign the __ktime_divns helper
>> to simplify the sign-handling logic, but then it
>> would likely just be a s64/s64 divide, and probably
>> should be more generic.
>>
>> Thoughts?
>
> Wouldn't it be better to simply forbid negative time altogether? Given
> it's been broken for quite a while, there must not be that many
> instances of such usage and fixing them would avoid the useless sign
> handling overhead to 99.9% of the cases.
Well, ktime is basically an s64 and timespecs can be negative as well.
So I'm not sure its reasonable to disqualify negative time intervals
from using this function. Especially since on 64bit systems,
ktime_divns handles negative intervals just fine.
>> Nicolas also notes that the ktime_divns() function
>> breaks if someone passes in a negative divisor as
>> well. This patch doesn't yet address that issue.
>
> GRanted, a negative divisor here would be even weirder and should
> definitely be rejected. Maybe the infinite loop is a good thing in that
> case, probably better than producing wrong numbers.
Yea. I'm thinking a WARN_ON or a BUG would be good to have in both
32bit and 64bit cases so we avoid folks testing on 64bit and thinking
it works generally.
thanks
-john
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] ktime: Fix ktime_divns to do signed division
2015-05-01 22:41 [PATCH] ktime: Fix ktime_divns to do signed division John Stultz
2015-05-01 22:43 ` John Stultz
2015-05-01 23:54 ` Nicolas Pitre
@ 2015-05-02 0:14 ` Nicolas Pitre
2015-05-02 8:31 ` Trevor Cordes
3 siblings, 0 replies; 6+ messages in thread
From: Nicolas Pitre @ 2015-05-02 0:14 UTC (permalink / raw)
To: John Stultz
Cc: linux-kernel, Thomas Gleixner, Josh Boyer, One Thousand Gnomes
On Fri, 1 May 2015, John Stultz wrote:
> static inline u64 ktime_divns(const ktime_t kt, s64 div)
> {
> if (__builtin_constant_p(div) && !(div >> 32)) {
> - u64 ns = kt.tv64;
> + s64 ns = kt.tv64;
> + int neg = 0;
> +
> + if (ns < 0) {
> + neg = 1;
> + ns = -ns;
> + }
Minor comment: you could save some realestate with:
s64 ns = kt.tv64;
bool neg = (ns < 0);
if (neg)
ns = -ns;
Nicolas
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] ktime: Fix ktime_divns to do signed division
2015-05-01 22:41 [PATCH] ktime: Fix ktime_divns to do signed division John Stultz
` (2 preceding siblings ...)
2015-05-02 0:14 ` Nicolas Pitre
@ 2015-05-02 8:31 ` Trevor Cordes
3 siblings, 0 replies; 6+ messages in thread
From: Trevor Cordes @ 2015-05-02 8:31 UTC (permalink / raw)
To: John Stultz
Cc: linux-kernel, Nicolas Pitre, Thomas Gleixner, Josh Boyer,
One Thousand Gnomes
On 2015-05-01 John Stultz wrote:
> It was noted that the 32bit implementation of ktime_divns
> was doing unsgined division adn didn't properly handle
> negative values.
>
> This patch fixes the problem by checking and preserving
> the sign bit, and then reapplying it if appropriate
> after the division.
I worked your new patch into my test system into Fedora 21's latest
3.19.5-200.fc21 (they haven't switched to 4.0 yet). I had to add in
Nicolas' 8b618628 commit as well to get this new patch to go into
3.19.5.
After rebuild and reboot I can confirm this patch fixes my bug: irsend
doesn't hang lircd. I'll report if anything else weird goes on, but
this all seems pretty straightforward so I doubt I'll have any problems.
> I'll send it out here shortly. If you could give it a spin at your
> leisure, and if it works give me a Tested-by: tag I'd appreciate it!
I'm not quite sure how to give a Tested-by, but from the minimal docs I
found on the net, I am trying below (after your Signed-off-by tag). If
I need to do something else, please point me in the general direction
of instructions.
> Great work again on chasing this down, and thanks for helping with
> debugging and validating the fix!
Thanks! I'm really glad this is getting worked out and am happy to
help. It's a big step forward for me to move past simple bugzilla onto
kernel bisects. I plan on giving a presentation about all this at my
local Linux user group. It'll be nice to report the happy ending! You
guys have been great. I guess kernel bugzilla isn't the place to get
help: it's here on the LKML.
P.S. Here's the kernel bz link, we/I can close it with the results once
this is all done:
https://bugzilla.kernel.org/show_bug.cgi?id=95431
> Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Josh Boyer <jwboyer@redhat.com>
> Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
> Reported-by: Trevor Cordes <trevor@tecnopolis.ca>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
Tested-by: Trevor Cordes <trevor@tecnopolis.ca> (runtime test on i686)
> ---
> include/linux/ktime.h | 12 ++++++++++--
> kernel/time/hrtimer.c | 11 +++++++++--
> 2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..d947263 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,12 +166,20 @@ static inline bool ktime_before(const ktime_t
> cmp1, const ktime_t cmp2) }
>
> #if BITS_PER_LONG < 64
> -extern u64 __ktime_divns(const ktime_t kt, s64 div);
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
> static inline u64 ktime_divns(const ktime_t kt, s64 div)
> {
> if (__builtin_constant_p(div) && !(div >> 32)) {
> - u64 ns = kt.tv64;
> + s64 ns = kt.tv64;
> + int neg = 0;
> +
> + if (ns < 0) {
> + neg = 1;
> + ns = -ns;
> + }
> do_div(ns, div);
> + if (neg)
> + ns = -ns;
> return ns;
> } else {
> return __ktime_divns(kt, div);
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..4c1b294 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,17 @@ lock_hrtimer_base(const struct hrtimer *timer,
> unsigned long *flags) /*
> * Divide a ktime value by a nanosecond value
> */
> -u64 __ktime_divns(const ktime_t kt, s64 div)
> +s64 __ktime_divns(const ktime_t kt, s64 div)
> {
> - u64 dclc;
> + s64 dclc;
> int sft = 0;
> + int neg = 0;
>
> dclc = ktime_to_ns(kt);
> + if (dclc < 0) {
> + neg = 1;
> + dclc = -dclc;
> + }
> /* Make sure the divisor is less than 2^32: */
> while (div >> 32) {
> sft++;
> @@ -279,6 +284,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
> }
> dclc >>= sft;
> do_div(dclc, (unsigned long) div);
> + if (neg)
> + dclc = -dclc;
>
> return dclc;
> }
^ permalink raw reply [flat|nested] 6+ messages in thread