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monitor: only run coroutine commands in qemu_aio_context

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monitor_qmp_dispatcher_co() runs in the iohandler AioContext that is not
polled during nested event loops. The coroutine currently reschedules
itself in the main loop's qemu_aio_context AioContext, which is polled
during nested event loops. One known problem is that QMP device-add
calls drain_call_rcu(), which temporarily drops the BQL, leading to all
sorts of havoc like other vCPU threads re-entering device emulation code
while another vCPU thread is waiting in device emulation code with
aio_poll().

Paolo Bonzini suggested running non-coroutine QMP handlers in the
iohandler AioContext. This avoids trouble with nested event loops. His
original idea was to move coroutine rescheduling to
monitor_qmp_dispatch(), but I resorted to moving it to qmp_dispatch()
because we don't know if the QMP handler needs to run in coroutine
context in monitor_qmp_dispatch(). monitor_qmp_dispatch() would have
been nicer since it's associated with the monitor implementation and not
as general as qmp_dispatch(), which is also used by qemu-ga.

A number of qemu-iotests need updated .out files because the order of
QMP events vs QMP responses has changed.

Solves Issue #1933.

Cc: qemu-stable@nongnu.org
Fixes: 7bed89958b ("device_core: use drain_call_rcu in in qmp_device_add")
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2215192
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2214985
Buglink: https://issues.redhat.com/browse/RHEL-17369
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240118144823.1497953-4-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2024-01-18 09:48:23 -05:00 • committed by Kevin Wolf
parent 9ee2dd4c22
commit effd60c878
32 changed files with 207 additions and 180 deletions
Download patch file Download diff file Expand all files Collapse all files

17
monitor/qmp.c
View file

@ -321,14 +321,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
qemu_coroutine_yield();
}
/*
* Move the coroutine from iohandler_ctx to qemu_aio_context for
* executing the command handler so that it can make progress if it
* involves an AIO_WAIT_WHILE().
*/
aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co);
qemu_coroutine_yield();
/* Process request */
if (req_obj->req) {
if (trace_event_get_state(TRACE_MONITOR_QMP_CMD_IN_BAND)) {
@ -355,15 +347,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
}
qmp_request_free(req_obj);
/*
* Yield and reschedule so the main loop stays responsive.
*
* Move back to iohandler_ctx so that nested event loops for
* qemu_aio_context don't start new monitor commands.
*/
aio_co_schedule(iohandler_get_aio_context(), qmp_dispatcher_co);
qemu_coroutine_yield();
}
qatomic_set(&qmp_dispatcher_co, NULL);
}