Messages in this thread | | | Date | Tue, 12 Jan 2016 12:48:54 +0000 | From | Nicholas Mc Guire <> | Subject | Re: [PATCH RFC] timer: drop the unnecessary while loop in msleep |
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On Mon, Jan 11, 2016 at 09:15:25AM +0100, Thomas Gleixner wrote: > On Sat, 9 Jan 2016, Nicholas Mc Guire wrote: > > > The while loop in msleep does not seem necessary as > > timeout is unsigned long and no larger than MAX_JIFFY_OFFSET (which is > > LONG_MAX/2 - 1) so the while-loop condition is always true at the beginning > > (msecs_to_jiffies will return >=0 always and with the +1 timeout is >= 1 so > > the while condition is always true at the start) and > > schedule_timeout_uninterruptible always returns 0, so the while loop always > > terminates after the first loop. > > Err, no. schedule_timeout_uninterruptible() can return > 0 when there was a > non timer wakeup. Thinks spurious wakeups. So we need that loop.
ok - thanks - was following the comment in schedule_timeout which states: /** * schedule_timeout - sleep until timeout * @timeout: timeout value in jiffies * * Make the current task sleep until @timeout jiffies have * elapsed. The routine will return immediately unless * the current task state has been set (see set_current_state()). * * You can set the task state as follows - * * %TASK_UNINTERRUPTIBLE - at least @timeout jiffies are guaranteed to * pass before the routine returns. The routine will return 0 <snip>
So I had assumed that would not actualy be possible given this comment. Is the while(timeout) loop im msleep() just defensive programming or is there a spurious timer wakeup path that defeats TASK_UNINTERRUPTIBLE ? Probably a stupid question but I was unable to figure out how such an wakeup would occure.
> > > Q: what is the purpose of the + 1 offset to the jiffies here ? > > > > msleep was introduced in 2.6.7 but without the + 1, so with: > > unsigned long timeout = msecs_to_jiffies(msecs); > > in 2.6.10-rc2 the msecs_to_jiffies(msecs) + 1; is introduced. > > Nishanth Aravamudan <nacc@us.ibm.com> (https://lkml.org/lkml/2004/11/19/294) > > seems to be the origin while converting msleep to a macro, but no reason > > for the + 1 is given there. > > Not really. The +1 was introduced with the following commit: > > https://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/commit/kernel/timer.c?id=c259ef842622a5e64418d9dab3b62ee051867edf >
thanks - was ignorant of history.git being available. will go try and understand where that "lost jiffie" could come from.
thx! hofrat
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