On Tue, 25 Oct 2011 15:21:11 +0200 Alon Levy <al...@redhat.com> wrote:
> On Tue, Oct 25, 2011 at 10:51:30AM -0200, Luiz Capitulino wrote: > > On Tue, 25 Oct 2011 12:13:09 +0200 > > Alon Levy <al...@redhat.com> wrote: > > > > > On Mon, Oct 24, 2011 at 10:31:48PM -0200, Luiz Capitulino wrote: > > > > On Mon, 24 Oct 2011 19:29:37 +0200 > > > > Alon Levy <al...@redhat.com> wrote: > > > > > > > > > On Mon, Oct 24, 2011 at 01:45:16PM -0200, Luiz Capitulino wrote: > > > > > > On Mon, 24 Oct 2011 17:13:14 +0200 > > > > > > Gerd Hoffmann <kra...@redhat.com> wrote: > > > > > > > > > > > > > On 10/24/11 14:02, Alon Levy wrote: > > > > > > > > Make screen_dump monitor command an async command to allow next > > > > > > > > for qxl > > > > > > > > to implement it as a initiating call to red_worker and > > > > > > > > completion on > > > > > > > > callback, to fix a deadlock when issueing a screendump command > > > > > > > > via > > > > > > > > libvirt while connected with a libvirt controlled spice-gtk > > > > > > > > client. > > > > > > > > > > > > > > Approach looks reasonable to me. Patch breaks the build though, > > > > > > > you've > > > > > > > missed a bunch of screen_dump functions in non-x86 targets. > > > > > > > > > > > > There are two problems actually. > > > > > > > > > > > > The first one is that changing an existing command from synchronous > > > > > > to asynchronous is an incompatible change because asynchronous > > > > > > commands > > > > > > semantics is different. For an example of possible problems please > > > > > > check: https://bugzilla.redhat.com/show_bug.cgi?id=623903. > > > > > > > > > > > > The second problem is that the existing asynchronous interface in > > > > > > the > > > > > > monitor is incomplete and has never been used for real. Our plan is > > > > > > to > > > > > > use QAPI's async support, but that has not landed in master yet and > > > > > > iirc > > > > > > there wasn't consensus about it. I also think it's a bit late for > > > > > > its > > > > > > inclusion in 1.0 (and certainly not a candidate for stable). > > > > > > > > > > > > If all you need here is to delay sending the response, then maybe > > > > > > the > > > > > > current interface could work (although I honestly don't trust it and > > > > > > regret not having dropped it). Otherwise our only choice would be to > > > > > > work on getting the QAPI async support merged. > > > > > > > > > > My problem is that the io thread keeps the global mutex during the > > > > > wait, > > > > > that's why the async monitor is perfect for what I want - this is > > > > > exactly what it does. > > > > > > > > Let's not mix internal implementation details with what we want as > > > > an external interface. > > > > > > > > Can't you just make a vga_hw_screen_dump() specific callback? > > > > > > > > > > I don't understand how that would help - if the monitor command doesn't > > > return (normal sync operation) then the mutex is never dropped, and any > > > callback won't change that. > > > > I'm trying to figure out a different solution. > > > > Our primary motivation for making a QMP command asynchronous must be to > > give clients the ability to keep issuing commands while "slow" commands > > are running. If that's not what you want nor your primary motivation, > > then you're probably taking the wrong approach. > > That sounds right, and it was what I assumed the async monitor > implementation would do, boy was I surprised to discover it doesn't do > any such thing, but what it does do is return early, allow *other* io > related events (select returns) to be handled, and keeps the serialized > only-one-command-ongoing monitor usage. Yes, if that turns out to be needed internally then we'll have to add such functionality (but probably not as part of QMP's async support iiuc). > > If that is what you want, then you'll have to add a new command, because > > changing from asynchronous to synchronous is an incompatible change _and_ > > you shouldn't use the current interface, because it's botched (actually, > > I think I'm going to drop it right now as my last series fixed its only > > user). > > > > This is not what I want. I understand of course that it is what one > would design an async monitor / api to allow (it was after all what I > thought async monitor support meant). > > > Using a botched interface that doesn't do what's supposed to do but happens > > to > > solve a bug as a side effect will very likely end in tears at some point in > > the future. > > > > Right, but it's the opposite of the current case. > > > Now, I did some research and found this description of the problem: > > > > """ > > In testing my patches for 'add_client' support with SPICE, I noticed > > deadlock in the QEMU/SPICE code. It only happens if I close a SPICE > > client and then immediately reconnect within about 1 second. If I > > wait a couple of seconds before reconnecting the SPICE client I don't > > see the deadlock. > > """ > > (http://lists.nongnu.org/archive/html/qemu-devel/2011-10/msg02599.html) > > > > Is this accurate? Why does it _work_ after 1 second? > > > > This is an unrelated bug, I know I said different on the thread but now > rereading the callstack it is the channel_event locking workaround - and > it is fixed by a spice-server only patch (which stops calling > channel_event from the worker thread). So, can you try to describe the problem you're having to someone who is not familiar with qlx and its locking? > > > > On the other hand, thinking a bit about the reference to 623903 baloon > > > bug, I don't see a problem: the change doesn't affect the semantics for > > > any other device except qxl, which I've tested. For any other device, > > > the only difference is that instead of: > > > > > > do_screen_dump call > > > device specific implementation > > > return > > > > > > It becomes > > > > > > do_screen_dump call > > > device specific implementation (not qxl) > > > callback called (always - not conditional, no one stores it except > > > qxl) > > > return > > > > > > > > > > > > I haven't looked at QAPI async support, but I > > > > > understand it's a bit in the future. > > > > > > > > Yes, it's not for the immediate term. > > > > > >
