linux-kernel - Re: Help in understanding the tasklets

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Message-ID: <4C22847F.80109@gmail.com>
Date:	Thu, 24 Jun 2010 00:02:39 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Hari LKML <hari.lkml@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Help in understanding the tasklets

On 06/19/2010 08:36 PM, Hari LKML wrote:
> After reading some of the books and the codes here is my understanding
> about the tasklet_action function
> 
> the function statrs some thing like this
> 
> 1. If the tasklet is not running in another processor set the Bit
> TASKLET_STATE_RUN;     <<if (tasklet_trylock(t))>>
> 2. clear the TASKLET_STATE_SCHED bit;       <<if (!test_and_clear_bit(TASKLET_
> STATE_SCHED, &t->state))>>
> 3. start the tasklet     <<(    t->func(t->data))>>
> 
> 
> Now when the tasklets function is running and the same interrupt is
> raised again but now handled by a different processor (in a SMP
> system) the tasklet is now scheduled in a different processor (because
> the TASKLET_STATE_SCHED is already cleared in step 2)
> and there are every chance of the same type of  taklet running
> concurently in different processors thus braking the promise that same
> types of tasklets do not ever run simultaniously.
> 
> I think i have misunderstood one or the other concept so please
> correct me and help me in understanding the Linux Kernel code more
> clearly.

But tasklet_trylock will return 0 when t->func is running on another
processor already because of step 1. Thus the tasklet gets to the end of
the tasklet list and is tried again later.

-- 
js
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