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Re: [Qemu-devel] [PATCH] event_notifier: move to top-level directory

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] event_notifier: move to top-level directory
Date: Tue, 27 Sep 2011 16:23:58 -0500
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On 09/27/2011 11:39 AM, Paolo Bonzini wrote:
On 09/27/2011 06:05 PM, Anthony Liguori wrote:
Actually, for posix-aio, we can just switch to using g_idle_add().
g_idle_add() uses g_source_attach which is thread safe. g_idle_add()
gives you a thread safe mechanism to defer a piece of work to the
main loop which is really what we want here.

For that, a bottom half would also do (apart that I am not sure it is
async-safe with TCG). In fact, that would make sense since all of
posix_aio_process_queue could become a bottom half.

Bottom halves are signal safe, not thread safe.

To make bottom halves thread safe, you would (in the very least) have to add some barriers when reading/writing the scheduling flag. I think it's much better to just use GIdle sources though.

This can actually be made to work with sync I/O emulation too by
having another GMainLoop in the sync I/O loop although I thought I
recalled a patch series to remove that stuff.

... which stuff? :)

The sync I/O emulation. Since sync I/O is done in block drivers, they can just use coroutine I/O instead of sync I/O.

Another GMainLoop in the sync I/O loop is problematic for
two reasons: 1) the sync I/O loop does not relinquish the I/O thread mutex,
which makes it very different from the outer loop; 2) a nested GMainLoop keeps
polling all the file descriptors in the outer loop, which requires you to cope
with reentrancy in those monitor commands that flush AIO.

Re: (2), you can use a separate aio GMainContext. The trick is that you need to keep a global current main context that switches when you enter the AIO main loop.

It's doable, but since I think we're on the verge of eliminating sync I/O emulation, that's probably a better path to pursue.


Anthony Liguori


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