Re: [Qemu-devel] [PATCH v2] block: avoid SIGUSR2

[Kevin Wolf <kwolf@redhat.com>]

Am 27.10.2011 15:57, schrieb Stefan Hajnoczi:
> On Thu, Oct 27, 2011 at 03:26:23PM +0200, Kevin Wolf wrote:
>> Am 19.09.2011 16:37, schrieb Frediano Ziglio:
>>> Now that iothread is always compiled sending a signal seems only an
>>> additional step. This patch also avoid writing to two pipe (one from signal
>>> and one in qemu_service_io).
>>>
>>> Work with kvm enabled or disabled. strace output is more readable (less syscalls).
>>>
>>> Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
>>
>> Something in this change has bad effects, in the sense that it seems to
>> break bdrv_read_em.
> 
> How does it break bdrv_read_em?  Are you seeing QEMU hung with 100% CPU
> utilization or deadlocked?

Sorry, I should have been more detailed here.

No, it's nothing obvious, it must be some subtle side effect. The result
of bdrv_read_em itself seems to be correct (return value and checksum of
the read buffer).

However instead of booting into the DOS setup I only get an error
message "Kein System oder Laufwerksfehler" (don't know how it reads in
English DOS versions), which seems to be produced by the boot sector.

I excluded all of the minor changes, so I'm sure that it's caused by the
switch from kill() to a direct call of the function that writes into the
pipe.

> One interesting thing is that qemu_aio_wait() does not release the QEMU
> mutex, so we cannot write to a pipe with the mutex held and then spin
> waiting for the iothread to do work for us.
> 
> Exactly how kill and qemu_notify_event() were different I'm not sure
> right now but it could be a factor.

This would cause a hang, right? Then it isn't what I'm seeing.

Kevin