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From: Kyle Moffett <mrmacman_g4@mac.com>
Subject: Re: Thread flags modified without set_thread_flag() (non atomically)
Date: Mon, 5 Mar 2007 11:30:49 -0500
To: Andrew Morton <akpm@linux-foundation.org>
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On Mar 01, 2007, at 17:41:38, Andrew Morton wrote:
> Well, it's unusual for code to want to test multiple bitflags in  
> the same operation.  Perhaps thread_info.flags is unusual in that  
> regard.
>
> But one can still do
>
> 	if (foo->flags & (1<<bar)|(1<<zot))
>
> Which can get to be a pain if it happens in a lot of places.  But  
> if it only happens in a few places, this seems a reasonable price  
> to pay, given the safety gains.
>
> Plus I'm _forever_ having to go into the header file to remember  
> whether REQ_RW is the bitmask or the bit offset.

Hey Andrew, I think you just fell victim to an order-of-operations  
bug;  '&' has a higher priority than '|' does, so:

   if ( (foo->flags & (1<<bar)) | (1<<zot))

Or:

   if ( dynamic_value | always_true_constant )

And as such this if statement is always true.  You'd need an extra  
pair of parentheses to make it correct.

Cheers,
Kyle Moffett