From 3e531f1d6e28940329d7e8e9bc018622cca53004 Mon Sep 17 00:00:00 2001
From: Laszlo Ersek <lersek@redhat.com>
Date: Thu, 5 Feb 2015 15:35:06 +0100
Subject: aio_notify(): force main loop wakeup with SIGIO (aarch64 host only)
Patchwork-id: 63729
O-Subject: [RHELSA qemu-kvm-rhev PATCH 1/2] aio_notify(): force main loop wakeup with SIGIO (aarch64 host only)
Bugzilla: 1184405
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: Richard Jones <rjones@redhat.com>
RH-Acked-by: Andrew Jones <drjones@redhat.com>
This should be happening, before the patch:
thread pool worker thread main iothread
------------------------- -------------
aio_notify()
|
write() ------------------[eventfd]---------------> g_poll()
But g_poll is not woken(). The notification is lost on aarch64/KVM, and we
don't know why.
The patch introduces the following (theoretically superfluous) additional
notification chain, activated under the same circumstances:
thread pool worker thread sigwait_compat() thread main iothread
------------------------- ----------------------- -------------
aio_notify()
|
kill() -----[SIGIO]-----> sigwait()
|
write() -----[pipe]---> g_poll()
(The SIGIO signal is generated for the entire process, but it can only be
consumed by the sigwait() call in sigwait_compat(), because that's the
only thread and section of code where SIGIO is unblocked.
After the main iothread wakes, it dispatches the sigfd_compat_info
received over the pipe to the sigfd_handler() function, which (seeing the
SIG_DFL disposition for SIGIO, which coincides with NULL) simply throws it
away.)
This patch should make no difference (beyond a minuscule performance hit),
but in practice it makes the hang go away, by reliably waking g_poll().
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
async.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/async.c b/async.c
index b4bf205..d910559 100644
--- a/async.c
+++ b/async.c
@@ -291,6 +291,9 @@ void aio_notify(AioContext *ctx)
if (ctx->notify_me) {
event_notifier_set(&ctx->notifier);
atomic_mb_set(&ctx->notified, true);
+#ifdef HOST_AARCH64
+ kill(getpid(), SIGIO);
+#endif
}
}
--
2.5.5