[PATCH RFC xserver] os: Add a mutex to protect io buffers

[Adam Jackson]

On Wed, 2017-02-22 at 16:50 +0100, Olivier Fourdan wrote:
> WriteToClient() can be called from XIChangeDeviceProperty() so from the
> InputThread which is a problem as it allocates and free the input and
> output buffers.
> 
> Add a new mutex to protect accesses to the input and output buffers from
> being accessed from different threads.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99164
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99887
> Signed-off-by: Olivier Fourdan <ofourdan at redhat.com>
> ---
>  RFC: This is probably sub-optimal and broken in many places, but it
>       seems to avoid the memory corruption (so far)... At least it's a
>       start, I guess.

It's certainly an improvement in correctness, I just hate it. ;)

One problem with this is there are some requests (GetImage in
particular) whose reply is split up into multiple calls to
WriteToClient, which means if an XICDP call happens on the input thread
while GetImage is writing, the events will be interleaved with the
reply data. In addition to corrupting the returned image, the client
will almost certainly choke and die while attempting to process the
next reply/event.

We can paper over that by fixing the WriteToClient callers to
(recursively) take the io lock as well. But doing so highlights a
design issue with this approach. GetImage replies are slow because
they're a lot of data, so now (if you hit the same XICDP path) you've
made the input thread _block_ until the request completes, and the
mouse will get stuck.

What I'd like to see is the events built asynchronously and flushed
from the main thread (possibly only if we can tell we're running on the
input thread).

- ajax