linux-kernel - Re: [RFC] timers, pointers to functions and type safety

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Message-ID: <20061204114851.GA25859@elte.hu>
Date:	Mon, 4 Dec 2006 12:48:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...l.org>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] timers, pointers to functions and type safety

* Al Viro <viro@....linux.org.uk> wrote:

> > This is going to make a lot of data structures smaller, when the 
> > timer_list is embedded in the structure itself and for the lot, 
> > which ignores the timer callback argument anyway.
> 
> container_of => still lousy type safety.  All over the sodding place.

the question is: which is more important, the type safety of a 
container_of() [or type cast], which if we get it wrong produces a 
/very/ trivial crash that is trivial to fix - or embedded timers data 
structure size all around the kernel? I believe the latter is more 
important.

and we could have a runtime debugging option to tie the type of the 
structure to the timer list entry. For example by using 
__builtin_classify_type(), sizeof() and offsetof() to fingerprint timer 
structs at init_timer time, and then checking for that at container_of() 
time - or something like that. In fact, gcc should really give us a 
better way to categorize types than __builtin_classify_type(). We could 
probably improve the situation by having some global registry of types 
known to the kernel, via a huge switch() around 
__builtin_types_compatible_p().

	Ingo
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