Re: question about preempt_disable()
From: Matthias Urlichs (smurf_at_smurf.noris.de)
Date: 11/30/03
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- In reply to: Chris Peterson: "question about preempt_disable()"
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To: linux-kernel@vger.kernel.org Date: Sun, 30 Nov 2003 18:39:48 +0100
Hi, Chris Peterson wrote:
> My question is: if the code is already SMP-safe and holding the necessary
> spinlocks, why is the preempt count necessary? Why must preemption be
> disabled and re-enabled as spinlocks are acquired and released?
You need to prevent deadlocks. Imagine process A grabbing a spinlock, then
getting preempted. Process B now sits there and waits on the spinlock.
Forward progress may or may not happen when the scheduler preempts B and
restarts A, some indeterminate time later.
Scheduling when waiting for a spinlock doesn't make sense because usually
the spinlock is held for just a few cycles (that's why it's a spin lock
and not a semaphore / wait queue / whatever), and rescheduling would take
more time than just waiting.
--
Matthias Urlichs | {M:U} IT Design @ m-u-it.de | smurf@smurf.noris.de
Disclaimer: The quote was selected randomly. Really. | http://smurf.noris.de
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You will have many recoverable tape errors.
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- Previous message: Jeff Garzik: "Re: Silicon Image 3112A SATA trouble"
- In reply to: Chris Peterson: "question about preempt_disable()"
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