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authorFam Zheng <famz@redhat.com>2018-08-09 21:22:59 +0800
committerFam Zheng <famz@redhat.com>2018-08-15 10:12:35 +0800
commitb37548fcd1b8ac2e88e185a395bef851f3fc4e65 (patch)
tree7553434946091fb4db57a90a1e5202a691827486 /util/aio-win32.c
parent70232b5253a3c4e03ed1ac47ef9246a8ac66c6fa (diff)
aio: Do aio_notify_accept only during blocking aio_poll
An aio_notify() pairs with an aio_notify_accept(). The former should happen in the main thread or a vCPU thread, and the latter should be done in the IOThread. There is one rare case that the main thread or vCPU thread may "steal" the aio_notify() event just raised by itself, in bdrv_set_aio_context() [1]. The sequence is like this: main thread IO Thread =============================================================== bdrv_drained_begin() aio_disable_external(ctx) aio_poll(ctx, true) ctx->notify_me += 2 ... bdrv_drained_end() ... aio_notify() ... bdrv_set_aio_context() aio_poll(ctx, false) [1] aio_notify_accept(ctx) ppoll() /* Hang! */ [1] is problematic. It will clear the ctx->notifier event so that the blocked ppoll() will not return. (For the curious, this bug was noticed when booting a number of VMs simultaneously in RHV. One or two of the VMs will hit this race condition, making the VIRTIO device unresponsive to I/O commands. When it hangs, Seabios is busy waiting for a read request to complete (read MBR), right after initializing the virtio-blk-pci device, using 100% guest CPU. See also https://bugzilla.redhat.com/show_bug.cgi?id=1562750 for the original bug analysis.) aio_notify() only injects an event when ctx->notify_me is set, correspondingly aio_notify_accept() is only useful when ctx->notify_me _was_ set. Move the call to it into the "blocking" branch. This will effectively skip [1] and fix the hang. Furthermore, blocking aio_poll is only allowed on home thread (in_aio_context_home_thread), because otherwise two blocking aio_poll()'s can steal each other's ctx->notifier event and cause hanging just like described above. Cc: qemu-stable@nongnu.org Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com> Message-Id: <20180809132259.18402-3-famz@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com>
Diffstat (limited to 'util/aio-win32.c')
-rw-r--r--util/aio-win32.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/util/aio-win32.c b/util/aio-win32.c
index e676a8d9b2..c58957cc4b 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -373,11 +373,12 @@ bool aio_poll(AioContext *ctx, bool blocking)
ret = WaitForMultipleObjects(count, events, FALSE, timeout);
if (blocking) {
assert(first);
+ assert(in_aio_context_home_thread(ctx));
atomic_sub(&ctx->notify_me, 2);
+ aio_notify_accept(ctx);
}
if (first) {
- aio_notify_accept(ctx);
progress |= aio_bh_poll(ctx);
first = false;
}