Re: [Qemu-devel] [RFC 09/10] cpu: remove exit_request global.

[Peter Maydell]

On 2 February 2015 at 10:03, Paolo Bonzini <address@hidden> wrote:
>> You can't do this -- this code is a signal handler so it could
>> get run at any time including while the list of CPUs is being
>> updated. (This is why we have the exit_request flag in the
>> first place rather than just setting the exit_request flag in
>> each CPU...)
>
> Actually you can do this if you are careful.
>
> In particular, you can do it while you are under the big QEMU lock.

...but this is a signal handler, so we can't guarantee that the
thread holds the big lock.

>  If
> you are not, basically you have to treat the CPU list as RCU-protected,
> and this is doable because the CPU object cannot be removed and added
> back into the CPU list.
>
> Unfortunately RCU doesn't support QTAILQ, at least not yet, so you'd
> have to convert the CPU list to QLIST.  But the basic idea of this patch
> can be done.

...and if we can iterate over CPU lists in signal handlers,
the correct approach is probably to use the existing exit_request
flag rather than adding another one. (Needs investigation to
check and document semantics of that flag.)

-- PMM