linux-kernel - Re: [patch] s390: do not use _local_bh

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Date:	Fri, 23 Feb 2007 14:41:37 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] s390: do not use _local_bh_enable()

On Fri, 2007-02-23 at 10:36 +0100, Heiko Carstens wrote:
> Same here: this is not really an irq handler but a function that gets called
> from different contexts and pretends to be an irq handler. The
> local_bh_disable()/_local_bh_enable() pair is just a trick to prevent bottom
> halve execution. I think you can blame Martin for this ;)

This code is really tricky. There are now three functions (cio_tpi,
sclp_sync_wait and __udelay) where we use a local_bh_disable()
_local_bh_enable() pair while we are disabled for interrupts. cio_tpi
and sclp_sync_wait are used to wait for the interrupt of the console
device to make room in the buffer for a printk out of disabled context.
__udelay() is used in the ETR clock-synchronization code where we are
disabled as well and the only alternative would be looping on a STCK for
about a second.

So basically we want to synchronously receive a specific interrupt. All
other interrupt sources are disabled. We know that the interrupt we wait
for will not cause a softirq to get scheduled. In case of sclp_sync_wait
and __udelay the interrupt is delivered the usual way by using the
asynchronous interrupt handler (I told you it is tricky ;-). That works
as long as only the hardirq part of the interrupt is executed, the
softirq part may not happen -> local_bh_disable. Back in the function
after the interrupt we cannot allow the softirq to happen when we
reenable the bottom-halves since the functions are called disabled and
need to stay disabled -> _local_bh_enable. Since the interrupt we waited
for did not add a softirq no harms is done, no?

-- 
blue skies,              IBM Deutschland Entwicklung GmbH
   Martin                Vorsitzender des Aufsichtsrats: Johann Weihen
                         Geschäftsführung: Herbert Kircher
Martin Schwidefsky       Sitz der Gesellschaft: Böblingen
Linux on zSeries         Registergericht: Amtsgericht Stuttgart,
   Development           HRB 243294

"Reality continues to ruin my life." - Calvin.

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