linux-kernel - Re: use of preempt_count instead of in

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Date:	Fri, 21 Mar 2008 18:02:31 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

PS,
I wrote:
> Tetsuo Handa wrote:
>> So, it is impossible to know whether I am inside a spinlock or not.
>> OK. That's not what I want to do.
>>
>> I want to make sure that my code (not a device driver) is called only 
>> from a context where use of down()/mutex_lock()/kmalloc(GFP_KERNEL)/
>> get_user_pages()/kmap() etc. are permitted.
>> Is "if (in_atomic()) return;" check a correct method for avoiding 
>> deadlocks when my code was accidentally called from a context
>> where use of down()/mutex_lock()/kmalloc(GFP_KERNEL)/get_user_pages()/
>> kmap() etc. are not permitted?

No.  Quoting Andrew:  "in_atomic() returns false inside spinlock on 
non-preemptible kernels."

> You shouldn't sleep while holding a spinlock.  As soon as another thread 

or interrupt handler or tasklet

> attempts to take the spinlock, it will be stuck in a busy-wait loop.
-- 
Stefan Richter
-=====-==--- --== =-=-=
http://arcgraph.de/sr/
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