Adding few more data points... On Fri, Aug 16, 2019 at 10:25 PM Arul Jeniston <arul.jeniston@xxxxxxxxx> wrote: > > Hi tglx, > > Thank you for your comments. > Please find my commend in-lined > > On Fri, Aug 16, 2019 at 4:15 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > > > Arul, > > > > On Fri, 16 Aug 2019, Arul Jeniston wrote: > > > > > Subject: [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function. > > > > The prefix is not 'FS: timerfd:' > > > > 1) The usual prefix for fs/* is: 'fs:' but... > > > > 2) git log fs/timerfd.c gives you a pretty good hint for the proper > > prefix. Look at the commits which actually do functional changes to that > > file, not at those which do (sub)system wide cleanups/adjustments. > > > > Also 'timerfd_read function' can be written as 'timerfd_read()' which > > spares the redundant function and clearly marks it as function via the > > brackets. > > > > > 'hrtimer_forward_now()' returns zero due to bigger backward time drift. > > > This causes timerfd_read to return 0. As per man page, read on timerfd > > > is not expected to return 0. > > > This problem is well explained in https://lkml.org/lkml/2019/7/31/442 > > > > 1) The explanation needs to be in the changelog itself. Links can point to > > discussions, bug-reports which have supplementary information. > > > > 2) Please do not use lkml.org links. > > > > Again: Please read and follow Documentation/process/submitting-patches.rst > > > > > . This patch fixes this problem. > > > Signed-off-by: Arul Jeniston <arul.jeniston@xxxxxxxxx> > > > > Missing empty line before Signed-off-by. Please use git-log to see how > > changelogs are properly formatted. > > > > Also: 'This patch fixes this problem' is not helpful at all. Again see the > > document I already pointed you to. > > > > Agreed. Would incorporate all the above comments. > > > > --- > > > fs/timerfd.c | 12 ++++++++++-- > > > 1 file changed, 10 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/timerfd.c b/fs/timerfd.c > > > index 6a6fc8aa1de7..f5094e070e9a 100644 > > > --- a/fs/timerfd.c > > > +++ b/fs/timerfd.c > > > @@ -284,8 +284,16 @@ static ssize_t timerfd_read(struct file *file, > > > char __user *buf, size_t count, > > > &ctx->t.alarm, ctx->tintv) - 1; > > > alarm_restart(&ctx->t.alarm); > > > } else { > > > - ticks += hrtimer_forward_now(&ctx->t.tmr, > > > - ctx->tintv) - 1; > > > + u64 nooftimeo = hrtimer_forward_now(&ctx->t.tmr, > > > + ctx->tintv); > > > > nooftimeo is pretty non-intuitive. The function documentation of > > hrtimer_forward_now() says: > > > > Returns the number of overruns. > > > > So the obvious variable name is overruns, right? > > > > Agreed. Would change the variable name to overruns. > > > > + /* > > > + * ticks shouldn't become zero at this point. > > > + * Ignore if hrtimer_forward_now returns 0 > > > + * due to larger backward time drift. > > > > Again. This explanation does not make any sense at all. > > > > Time does not go backwards, except if it is CLOCK_REALTIME which can be set > > backwards via clock_settime() or settimeofday(). > > > > > + */ > > > + if (likely(nooftimeo)) { > > > + ticks += nooftimeo - 1; > > > + } > > > > Again: Pointless brackets. > > > > If you disagree with my review comment, then tell me in a reply. If not, > > then fix it. If you decide to ignore my comments, then don't wonder if I > > ignore your patches. > > > > We use CLOCK_REALTIME while creating timer_fd. > Can read() on timerfd return 0 when the clock is set to CLOCK_REALTIME? > > We have Intel rangely 4 cpu system running debian stretch linux > kernel. The current clock source is set to tsc. During our testing, we > observed the time drifts backward occasionally. Through kernel > instrumentation, we observed, sometimes clocksource_delta() finds the > current time lesser than last time. and returns 0 delta. > This causes the following code flow to return a time which is lesser than previously fetched time. ktime_get()-->timekeeping_get_ns()-->timekeeping_get_delta()-->clocksource_delta() Since ktime_get() returns a time which is lesser than the expiry time, hrtimer_forward_now return 0. This in-turn causes timerfd_read to return 0. Is it not a bug? > > Thanks, > > > > tglx
