linux-kernel - Re: Thread flags modified without set_thread

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Message-Id: <1ED05FB5-17F7-42B2-BEAA-446F7DADAE46@mac.com>
Date:	Mon, 5 Mar 2007 11:30:49 -0500
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mathieu Desnoyers <compudj@...gle.com>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Thread flags modified without set_thread_flag() (non atomically)

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

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