linux-kernel - Re: Lots of bugs with current->state = TASK

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Message-ID: <4B58B759.8000002@caviumnetworks.com>
Date:	Thu, 21 Jan 2010 12:21:45 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	rostedt@...dmis.org
CC:	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors <kernel-janitors@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, Greg KH <greg@...ah.com>,
	Andy Whitcroft <apw@...onical.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	linux-mips <linux-mips@...ux-mips.org>
Subject: Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

>>
>> This is what I thought.
>>
>> My cpu (Cavium Octeon) does not have out of order reads, so my wmb() is 
> 
> Can you have reads that are out of order wrt writes? Because the above
> does not have out of order reads. It just had a read that came before a
> write. The above code could look like:
> 
> (hypothetical assembly language)
> 
> 	ld r2, TASK_UNINTERRUPTIBLE
> 	st r2, (current->state)
> 	wmb
> 	ld r1, (x)
> 	cmp r1, 0
> 
> Is it possible for the CPU to do the load of r1 before storing r2? If
> so, then the bug still exists.
> 

Indeed it is.  Lockless operations make my head hurt.

Thanks for clarifying.

David Daney



> -- Steve
> 
> 
>> in fact a full mb() from the point of view of the current CPU.  So I 
>> think I could weaken my bariers in set_current_state() and still get 
>> correct operation.  However as you say...
>>
> 
> 
> 

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