linux-kernel - TFD_CANCEL_ON_SET race when making a wall clock

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Message-ID: <1314974233.30505.9.camel@lenny>
Date:	Fri, 02 Sep 2011 10:37:12 -0400
From:	Colin Walters <walters@...bum.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: TFD_CANCEL_ON_SET race when making a wall clock

Hi,

So I was recently making GNOME use the new timerfd TFD_CANCEL_ON_SET so
we get woken up when the system clock changes.  It works generally well,
except Ryan Lortie pointed out a race condition in my use of
timerfd_settime() that I think anyone using it to make a wall clock
display might not realize at first:

https://bugzilla.gnome.org/show_bug.cgi?id=655129#c36

For the link-averse, basically the system clock can move backwards
between when the process gets the current time, and computes the wakeup
(typically for the next minute).

I was able to work around it in userspace with this patch:
http://bugzilla-attachments.gnome.org/attachment.cgi?id=195252
But it's clearly not what I'd call beautiful.

I don't see a nice way to handle this in the kernel given the current
API, but maybe someone else does?

TFD_CANCEL_ON_SET isn't documented in man-pages at all right now...maybe
this is just a useful note for a future patch to
man-pages/man2/timerfd_create.2.

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