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    SubjectRe: Extending virtio_console to support multiple ports
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    On Wed, 2009-08-26 at 21:15 +0530, Amit Shah wrote:

    >
    > > - 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.

    Two things here:

    - First you seem to have completely missed the fact that hvc_poll() can
    be called from interrupt time :-) Look at hvc_irq.c which is used by
    some backends. Maybe that can be "fixed" by deferring to a work queue,
    though it's nice to have the keyboard input have somewhat of a higher
    priority than anything else here.

    So unless that's fixed, or I missed something, that's a big NACK for
    now.

    - Then, are we certain that there's no case where the tty layer will
    call us with some lock held or in an atomic context ? To be honest, I've
    totally lost track of the locking rules in tty land lately so it might
    well be ok, but something to verify.

    Cheers,
    Ben.




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