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Re: [Xen-devel] [PATCH] add netconsole support for xen-netfront

On 1/18/2012 12:59 AM, Ian Campbell wrote:
On Tue, 2012-01-17 at 23:15 +0000, Tina Yang wrote:
On 1/17/2012 1:51 PM, Konrad Rzeszutek Wilk wrote:
On Tue, Jan 17, 2012 at 01:42:22PM -0800, Tina Yang wrote:
On 1/13/2012 3:06 AM, Ian Campbell wrote:
Although netdump is now obsolete, I think it's always a good practice
to preserve caller's irq status as we had a very bad experience
chasing a similar problem caused by such a irq change in RDS
Did you find the culprit of it? Was there a patch for that in the
upstream kernel?
Yes.  It has nothing to do with net drivers but same cause
elsewhere in the kernel.
I didn't think start_xmit could be called with interrupts disabled or
from interrupt context but perhaps I am wrong about that or perhaps
netconsole changes things?
Netdump does call it with interrupt disabled and hang because of
it in 2.6.9 as I remember it.  And you are right, netconsole has
undergone changes from time to time, which also can change
this specification.

Right, Documentation/networking/netdevices.txt states that start_xmit
can be called with interrupts disabled by netconsole and therefore using
the irqsave/restore locking in this function is, AFAICT, correct.

in the not too long ago past.
OK, it sounds like it was issues in the past but might not be the
case anymore.

Could please re-test it without that spinlock irqsave patch using
the upstream kernel (or just UEK2 since it is an 3.0 type kernel).
Shouldn't be the case now, but don't know about the future.
The fact is as long as there is a new caller that has the expectation
of preserved irq status, it would be a problem.
The question is not so much what may or may not be a problem in the
future but what the requirements of this function are, in particular
those imposed by the network stack for the start_xmit function.
Agreed. It's not safe to assume it unless the API documentation states that it can
not be called with interrupt disabled explicitly.
As Ian said, some net drivers have been cautious in this regard already by
saving/restoring the status, but apparently not everyone.
I was talking about the interrupt/poll handler here since I hadn't yet
noticed that the locking change was also in start_xmit and not just the
poll/interrupt paths (which was actually just code motion and not a
locking change in any case).
I did look at the start_xmit of most of the net drivers myself when I hit the
netdump hang back in 2008, and some of them did save/restore already,
but others didn't.

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