@@ -591,6 +591,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
* so disable the optimization now.
*/
if (blocking) {
+ assert(in_aio_context_home_thread(ctx));
atomic_add(&ctx->notify_me, 2);
}
@@ -633,6 +634,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
if (blocking) {
atomic_sub(&ctx->notify_me, 2);
+ aio_notify_accept(ctx);
}
/* Adjust polling time */
@@ -676,8 +678,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
}
}
- aio_notify_accept(ctx);
-
/* if we have any readable fds, dispatch event */
if (ret > 0) {
for (i = 0; i < npfd; i++) {
@@ -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;
}
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> --- util/aio-posix.c | 4 ++-- util/aio-win32.c | 3 ++- 2 files changed, 4 insertions(+), 3 deletions(-)