linux-kernel - Re: [PATCH v2] add time_now_after and other macros which compare with jiffies

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Date:	Sun, 9 Mar 2008 11:36:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Dave Young" <hidave.darkstar@...il.com>,
	"Johannes Weiner" <hannes@...urebad.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] add time_now_after and other macros which compare
 with jiffies

On Sun, 9 Mar 2008 09:58:02 +0000 Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> > >  > +/* time_now_before_eq(a) return true if now (jiffies) is before or equal to a */
> > >  > +#define time_now_before_eq(a) time_before_eq(jiffies, a)
> > >
> > >  How about even more obvious names like time_is_past(), time_is_future(),
> > >  ...?
> > 
> > Thanks for comment.
> > 
> > Then how do we name the _eq version?  IMHO, the time_now_* is enough.
> 
> Why do you even need them. I don't see the point of *any* of these extra
> macros as they simply obfuscate code and hide what is actually going on.

Two reasons:

a) the existing macros are (I believe) a right royal pita.  It's very
   hard to remember which order the args are supposed to be in.

   So each time I see a time_foo() when reviewing a patch I have to go
   off and re-read the implementation then have a big think to check that
   they got it right (a sure sign of a poor interface, no)?

   And I'm not the only one - people get this wrong fairly regularly.

b) around 90% of the usages of time_after() are to compare against jiffies!

The macros which Dave is developing aren't just less-error-prone,
easier-to-review transformations - they offer higher-level functionality.
Because time_after() is just a basic comparison operation, whereas
time_now_before() is an *application* of that operation.

Trust me on this - they will lead to easier-to-review code and less bugs.

> The initial macros were added because of the type safety and correct
> comparison rules being complex. They have a purpose.

They are hard to use and hard to review.

> Even if you want these you can use !time_future() if you don't want the
> _eq variants. Generally speaking drivers should be using timers not
> polled loops, and most of our loops comparing with jiffies want removing.

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