Messages in this thread | | | Subject | Re: [patch] s390: do not use _local_bh_enable() | From | Martin Schwidefsky <> | Date | Fri, 23 Feb 2007 14:41:37 +0100 |
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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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