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From: Thomas Gleixner <tglx@linutronix.de>
To: Arul Jeniston <arul.jeniston@gmail.com>
Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, arul_mc@dell.com
Subject: Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function.
Date: Thu, 5 Sep 2019 17:34:29 +0200 (CEST)
Message-ID: <alpine.DEB.2.21.1909051707150.1902@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CACAVd4gRoQih6f_K7kMzr=AwA_DvP0OksxBKj1bGPsP2F_9sFg@mail.gmail.com>

Arul,

On Thu, 5 Sep 2019, Arul Jeniston wrote:
> When we adjust the date setting using date command we observed
> 'timerfd_read()' on CLOCK_REALTIME (TFD_TIMER_ABSTIME flag is set)
> returns 0.
> we don't see any hardware influence here and we are able to recreate
> it consistently. Is it expected? if yes, isn't it something to be
> documented in timerfd read() man page?

It's expected, yes. Simply because it hits the following condition:

     armtimer(T1)

     settime(T1 + X)  --> causes timer to fire

     		      	  	 wakeup reader
     settime(T0)

				 read number of intervals: 0

				 i.e. timer did not expire

Changing the return value to 1 would be just a cosmetic workaround. We
could also jump back and wait again. But that's all not consistent because

     armtimer(T1)

     settime(T1 + X)  --> causes timer to fire

     		      	  	 wakeup reader

				 read number of intervals: 1
     settime(T0)

				 user space reads time and figures that
				 the returned tick is bogus.

So I rather change the documentation (this applies only to CLOCK_REALTIME
and CLOCK_REALTIME_ALARM) and explain the rationale.

For applications which care about notifications when the time was set,
timerfd_settime() provides TFD_TIMER_CANCEL_ON_SET which causes the timer
to be canceled when time is set and returns -ECANCELED from the
read. That's unambiguous.

Thanks,

	tglx

  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16  8:32 [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function. '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 patch fixes this problem. Signed-off-by: Arul Jeniston <arul.jeniston@gmail.com> arul.jeniston
2019-08-16  9:05 ` [PATCH] FS: timerfd: [Trimmed unreadable long subject line ] Thomas Gleixner
2019-08-16 10:22 ` [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function Arul Jeniston
2019-08-16 10:45   ` Thomas Gleixner
2019-08-16 16:55     ` Arul Jeniston
2019-08-16 17:00       ` Arul Jeniston
2019-08-16 21:17         ` Thomas Gleixner
     [not found]           ` <CACAVd4hT6QYtgtDsBcgy7c_s9WVBAH+1m0r5geBe7BUWJWYhbA@mail.gmail.com>
2019-08-17 19:23             ` Thomas Gleixner
2019-08-19  6:07               ` Arul Jeniston
2019-08-19  8:04                 ` Thomas Gleixner
2019-08-19 14:25                   ` Arul Jeniston
2019-08-19 14:52                     ` Thomas Gleixner
2019-08-19 15:26                       ` Arul Jeniston
2019-08-19 15:59                         ` Thomas Gleixner
     [not found]                           ` <CACAVd4iRN7=eq_B1+Yb-xcspU-Sg1dmMo_=VtLXXVPkjN1hY5Q@mail.gmail.com>
2019-08-19 18:29                             ` Thomas Gleixner
     [not found]                               ` <CACAVd4jAJ5QcOH=q=Q9kAz20X4_nAc7=vVU_gPWTS1UuiGK-fg@mail.gmail.com>
2019-08-20  8:49                                 ` Thomas Gleixner
2019-08-20  9:42                                   ` Arul Jeniston
2019-09-05  8:48                                     ` Arul Jeniston
2019-09-05 15:34                                       ` Thomas Gleixner [this message]
2019-09-06 16:36                                         ` Arul Jeniston
2019-09-07 14:38                                           ` Thomas Gleixner
2019-11-05  5:01                                             ` Arul Jeniston
2019-11-05 10:01                                               ` Thomas Gleixner
2019-11-06  3:38                                                 ` Arul Jeniston

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