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From: Benjamin Herrenschmidt on 4 Aug 2010 04:40
On Wed, 2010-08-04 at 00:59 -0700, Dmitry Torokhov wrote:
> Noone is using tty argument so let's get rid of it.

While talking about sysrq...

I noticed that serial drivers call sysrq with the uart lock held.

I've seen deadlocks caused by that, for example, when entering the
debugger, it waits forever for a CPU which is itself waiting on the UART
lock.

I have a patch to drop the lock in serial_core.h, I'll post that
tomorrow hopefully, just checking if there's any objection there ? The
serial drivers might need to be audited a bit to make sure they cope
with the lock being dropped and re-acquired around the sysrq call.

Cheers,
Ben.


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From: Alan Cox on 4 Aug 2010 05:00
> I have a patch to drop the lock in serial_core.h, I'll post that
> tomorrow hopefully, just checking if there's any objection there ? The

Fundamentally - no. However the impact it has on a lot of the drivers
will be significant and you'll be submitting a huge patch pile to fix up
all the locking assumptions (for one it means port->tty might change
across any call that ends up in sysrq)

> serial drivers might need to be audited a bit to make sure they cope
> with the lock being dropped and re-acquired around the sysrq call.

Architecturally I think it would make more sense to add a new sysrq
helper which merely sets a flag, and check that flag at the end of the IRQ
when dropping the lock anyway.

Otherwise it'll be a huge amount of work to even build test all those
consoles.

Alan
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From: Benjamin Herrenschmidt on 4 Aug 2010 22:10
On Wed, 2010-08-04 at 07:03 -0500, Jason Wessel wrote:
> The usb serial has the same kind of problem with all sorts of locks.
> I
> had resorted to using a tasklet or workqueue to get the sysrq to
> execute
> just outside the usb serial driver context. This work never made it
> upstream because it was part of another series, but the reference is:
> http://lkml.org/lkml/2010/3/9/17
>
> For the standard serial you might be able to get away with dropping
> the
> lock for the sysrq.
>
> Did you have a particular way to trigger the problem or was it just
> completely random, because I don't know that I have observed this
> behavior with the typical 8250 driver.

Yes, I do actually :-) A 64-way machine and a bunch of processes
flooding the console will do it just fine :-)

Cheers,
Ben.


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