Re: [PATCH 1/2] BDI: Provide backing device capability information

[Linus Torvalds]

On Wed, 2 Mar 2005, Andrew Morton wrote:
>
> Why not make these bitfields as well?

Side note: bitfields aren't exactly wonderful. They tend to generate worse
code, and they make it much harder to work with combinations of flags
(both testing and initializing). They also have architecture-specific
bit-order and packing issues, which means that when coupled with device
drivers etc they can be a major pain in the derriere.

Even apart from those issues, they can't sanely be atomically accessed, so
in many cases they just aren't a good thing.

In contrast, just using a flag word and explicit bitmask has none of those 
problems, and it's often easy to abstract out things with a macro and/or 
inline function.

So don't go for bitfields "just because". It's generally a good idea to 
_require_ that the code in question has none of the potential problem 
spots even in _theory_, and in addition also show that bitfields really 
make the code look nicer. The downsides really can be that nasty.

		Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/