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SubjectSlow PPP, SCSI, and spinlocks
Date
Ok, this is really a follow up to my previous Slow PPP problem with my PS/2
at home. I had originally traced this to the 2.1.102 patch where the
cli/sti routines were converted to save_flags/restore_flags. Thanks to
input from another user who pointed out that the most likely scenario here
was that the cli/sti routine was inadvertenly renabling interrupts that had
been disabled by other code I pushed further into my studies of the irq
handling in Linux 2.2. Sure enough, the problem actually stems from the
change to wrap the SCSI interrupts handlers with spinlocks, basically
meaning that all interrupts remain disabled during the entire interrupt
handling routine, and by sheer luck the IBM SCSI driver contained a cli/sti
routine that reenabled interrupts causing my serial port to continue to
function properly until 2.1.102 when the routine was replaced by
save_flags/restore_flags.

I then tested this more completely by using the stock 2.2.1 IBM SCSI driver
with only one change, I removed the spin_lock_irqsave/spin_lock_irqrestore
from the do_interrupt_hanlder routine. With this minor change I no longer
loose serial interrupts.

The most information I could find about this change was in Linus' comments
about the 2.1.95 patch, and by my limited interpretation the changes were
mainly for SMP. So, my big question then is this, is it really neccessary
to do spin_locks on SCSI interrupt handlers even on UP machines? So far
I've noticed no ill effects with my change.

The problem with this seems to occur on machines other than mine. I
received several emails from other users who were experiencing similar
trouble with the Seagte scsi driver.

I'd like to hear some other comments on this. Perhaps there are better ways
to handle this such as trimming down the interrupt handler and moving some
portions to another routine that will funtion with interrupts enabled.

Any help would be greatly appreciated.

Later,
Tom



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