linux-kernel - Re: [PATCH] timer: Fix jiffies wrap behavior of round

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Message-Id: <20130529215005.c91c75de.akpm@linux-foundation.org>
Date:	Wed, 29 May 2013 21:50:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Bart Van Assche <bart.vanassche@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH] timer: Fix jiffies wrap behavior of round_jiffies*()

On Wed, 29 May 2013 17:13:49 -0700 Joe Perches <joe@...ches.com> wrote:

> On Wed, 2013-05-29 at 16:38 -0700, Andrew Morton wrote:
> > On Wed, 29 May 2013 16:17:47 -0700 Joe Perches <joe@...ches.com> wrote:
> > 
> > > >   We could perhaps have a checkpatch rule
> > > > which looks for comparisons against jiffes (and any other
> > > > time-measuring variables we can detect)
> > > 
> > > other variables like?
> > 
> > Grepping for time_after finds a bunch.  There's no real pattern to it though.
> 
> get_jiffies_64() should probably be added as
> another test too.
> 
> Also, these might be wrong:
> 
> arch/arm/kernel/smp_twd.c:              while (get_jiffies_64() < waitjiffies)
> arch/arm/kernel/smp_twd.c:              while (get_jiffies_64() < waitjiffies)
> fs/fuse/dir.c:  else if (fuse_dentry_time(entry) < get_jiffies_64()) {
> fs/fuse/dir.c:  if (fi->i_time < get_jiffies_64()) {
> fs/fuse/dir.c:          if (fi->i_time < get_jiffies_64()) {
> 

Yup.  Normally a 64-bit jiffy will wrap around shortly after the heat
death of the universe, but

a) it's derived from jiffies, which we evilly cause to wrap after 5
   minutes uptime and

b) it's derived from jiffies, which is 32-bit on 32-bit and hence
   wraps every 49 days (HZ=1000).

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