linux-kernel - Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd

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Message-ID: <alpine.DEB.2.21.1909051707150.1902@nanos.tec.linutronix.de>
Date:   Thu, 5 Sep 2019 17:34:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arul Jeniston <arul.jeniston@...il.com>
cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, arul_mc@...l.com
Subject: Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read
 function.

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