linux-kernel - Re: newbie questions about while (1) in kernel mode and spinlocks

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Date:	Thu, 21 Dec 2006 11:05:51 +0100
From:	Paolo Ornati <ornati@...twebnet.it>
To:	"Sorin Manolache" <sorinm@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: newbie questions about while (1) in kernel mode and spinlocks

On Thu, 21 Dec 2006 10:41:44 +0100
"Sorin Manolache" <sorinm@...il.com> wrote:

> The Linux Device Drivers book says that a spin_lock should not be
> shared between a process and an interrupt handler. The explanation is
> that the process may hold the lock, an interrupt occurs, the interrupt
> handler spins on the lock held by the process and the system freezes.
> Why should it freeze? Isn't it possible for the interrupt handler to
> re-enable interrupts as its first thing, then to spin at the lock, the
> timer interrupt to preempt the interrupt handler and to relinquish
> control to the process which in turn will finish its critical section
> and release the lock, making way for the interrupt handler to
> continue.

Iterrupt handlers are executend in the process context (on top of the
process that they interrupted).

So, if you have a proccess A that does:

	Usual Kernel Code		Interrupt Handler

	...
	spin_lock(my_lock);
	...
		-------interrupt----->	...
					spin_lock(my_lock); // deadlock!
					...
		<------ back --------- 
	----
	spin_unlock(my_lock);


See?

If the interrupt comes in when process A is running and holding the
lock PREEMPTION can't do anything.

-- 
	Paolo Ornati
	Linux 2.6.20-rc1-g99f5e971 on x86_64
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