On (Thu) Aug 27 2009 [15:04:45], Michael Ellerman wrote:

On Wed, 2009-08-26 at 21:15 +0530, Amit Shah wrote:

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[cc'ing some people who have made some commits in hvc_console.c]

On (Wed) Aug 26 2009 [16:57:18], Amit Shah wrote:

quoted

On (Tue) Aug 25 2009 [11:47:20], Amit Shah wrote:

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Hello all,

Here is a new iteration of the patch series that implements a
transport for guest and host communications.

The code has been updated to reuse the virtio-console device instead
of creating a new virtio-serial device.

And the problem now is that hvc calls the put_chars function with
spinlocks held and we now allocate pages in send_buf(), called from
put_chars.

A few solutions:

[snip]

quoted

- Convert hvc's usage of spinlocks to mutexes. I've no idea how this
  will play out; I'm no expert here. But I did try doing this and so far
  it all looks OK. No lockups, lockdep warnings, nothing. I have full
  debugging enabled. But this doesn't mean it's right.

So just to test this further I added the capability to have more than
one hvc console spawn from virtio_console, created two consoles and did
a 'cat' of a file in each of the virtio-consoles. It's been running for
half an hour now without any badness. No spew in debug logs too.

I also checked the code in hvc_console.c that takes the spin_locks.
Nothing there that runs from (or needs to run from) interrupt context.
So the change to mutexes does seem reasonable. Also, the spinlock code
was added really long back -- git blame shows Linus' first git commit
introduced them in the git history, so it's pure legacy baggage.

I won't tell Ryan you called his code "pure legacy baggage" if you
don't ;)

http://git.kernel.org/?p=linux/kernel/git/torvalds/old-2.6-bkcvs.git;a=commitdiff;h=d450b4ae023fb4be175389c18f4f87677da03020

Thanks for the link!

(and this general area might be the one that doesn't get major upheavals
in 5-yr spans :-)

		Amit