netdev - Re: [Question] Kernel preemption of BH handler

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Date:	Mon, 26 Mar 2012 15:56:14 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Erik Hugne <erik.hugne@...csson.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Question] Kernel preemption of BH handler

On Mon, 2012-03-26 at 11:20 +0200, Erik Hugne wrote:
> I have a BH handler that processes packets received from a netdevice.
> 
> my_bh_handler() {
>    spin_lock_bh(my_lock);
>    /*do stuff*/
>    spin_unlock_bh(my_lock);
>    /*do more stuff*/
> }

spin_lock_bh() means 'spin lock with contending soft-interrupts' and not
'spin lock from a soft-interrupt'.  You probably need to use it in
process context but not here.

> First packet is received, and my_bh_handler() is currently processing it.
> Now a new packet is received by the NIC and my_bh_handler() is preempted 
> after my_lock have been released.

You don't explain how this handler is invoked, but normally it would be
scheduled to run a second time and would not preempt the first call.

Ben.

> Is it possible that the second invocation of the BH routine is allowed 
> to finish before the first?

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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