Re: [Qemu-devel] [PATCH] rbd: switch from pipe to QEMUBH completion notification

[Josh Durgin <josh.durgin@inktank.com>]

On 12/05/2013 07:38 AM, Stefan Hajnoczi wrote:
> rbd callbacks are called from non-QEMU threads.  Up until now a pipe was
> used to signal completion back to the QEMU iothread.
>
> The pipe writer code handles EAGAIN using select(2).  The select(2) API
> is not scalable since fd_set size is static.  FD_SET() can write beyond
> the end of fd_set if the file descriptor number is too high.  (QEMU's
> main loop uses poll(2) to avoid this issue with select(2).)
>
> Since the pipe itself is quite clumsy to use and QEMUBH is now
> thread-safe, just schedule a BH from the rbd callback function.  This
> way we can simplify I/O completion in addition to eliminating the
> potential FD_SET() crash when file descriptor numbers become too high.
>
> Crash scenario: QEMU already has 1024 file descriptors open.  Hotplug an
> rbd drive and get the pipe writer to take the select(2) code path.
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> Josh: This patch has not been tested.  I have just compiled it.

Sorry for the delay. This is a great simplification, and it passed my
tests. Thanks!

Reviewed- and Tested-by: Josh Durgin <josh.durgin@inktank.com>