Re: [Qemu-devel] [PATCH] eventfd: making it thread safe

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 18, 2012 at 10:08:53PM +1000, Alexey Kardashevskiy wrote:
> QEMU uses IO handlers to run select() in the main loop.
> The handlers list is managed by qemu_set_fd_handler() helper
> which works fine when called from the main thread as it is
> called not when select() is waiting.

when select() is not waiting?

> 
> However IO handlers list can be changed in the thread other than
> the main one doing os_host_main_loop_wait(), for example, as a result
> of a hypercall which changes PCI config space (VFIO on POWER is the case)

So the problem is only with VFIO? Can it affect vhost-net?

> and enables/disabled MSI/MSIX which closes/creates eventfd handles.

There doesn't seem to be a notification in case an fd is
deleted. It's probably not at all urgent to remove
an fd from select - why do you mention closing handles?

> If the main loop is waiting on such eventfd, it has to be restarted.

Do you really mean 'should be waiting on the newly created
eventfd'?

> The patch adds the qemu_notify_event() call to interrupt select()
> and make main_loop() to restart select()

s/and make main_loop() to restart/to make main_loop() restart/?

> with the updated IO
> handlers list.
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  iohandler.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/iohandler.c b/iohandler.c
> index 3c74de6..dea4355 100644
> --- a/iohandler.c
> +++ b/iohandler.c
> @@ -77,6 +77,7 @@ int qemu_set_fd_handler2(int fd,
>          ioh->fd_write = fd_write;
>          ioh->opaque = opaque;
>          ioh->deleted = 0;
> +        qemu_notify_event();
>      }
>      return 0;
>  }
> -- 
> 1.7.10.4