From: Matthias Urlichs <smurf@smurf.noris.de>
To: linux-kernel@vger.kernel.org
Subject: Re: question about preempt_disable()
Date: Sun, 30 Nov 2003 18:39:48 +0100 [thread overview]
Message-ID: <pan.2003.11.30.17.39.47.71027@smurf.noris.de> (raw)
In-Reply-To: 000d01c3b6dd$30ab34a0$8a04a943@bananacabana
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
- -
You will have many recoverable tape errors.
next prev parent reply other threads:[~2003-11-30 17:40 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-11-30 0:59 question about preempt_disable() Chris Peterson
2003-11-30 17:39 ` Matthias Urlichs [this message]
2003-12-01 3:53 ` Rob Love
2003-12-08 16:31 ` Matthias Urlichs
2003-12-03 23:12 ` George Anzinger
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