[RFC 0/3] qmp: make qmp_device_add() a coroutine

[Stefan Hajnoczi]

It is not safe to call drain_call_rcu() from qmp_device_add() because
some call stacks are not prepared for drain_call_rcu() to drop the Big
QEMU Lock (BQL).

For example, device emulation code is protected by the BQL but when it
calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the
BQL is dropped. See https://bugzilla.redhat.com/show_bug.cgi?id=2215192 for a
concrete bug of this type.

Another limitation of drain_call_rcu() is that it cannot be invoked within an
RCU read-side critical section since the reclamation phase cannot complete
until the end of the critical section. Unfortunately, call stacks have been
seen where this happens (see
https://bugzilla.redhat.com/show_bug.cgi?id=2214985).

This patch series introduces drain_call_rcu_co(), which does the same thing as
drain_call_rcu() but asynchronously. By yielding back to the event loop we can
wait until the caller drops the BQL and leaves its RCU read-side critical
section.

Patch 1 changes HMP so that coroutine monitor commands yield back to the event
loop instead of running inside a nested event loop.

Patch 2 introduces the new drain_call_rcu_co() API.

Patch 3 converts qmp_device_add() into a coroutine monitor command and uses
drain_call_rcu_co().

I'm sending this as an RFC because I don't have confirmation yet that the bugs
mentioned above are fixed by this patch series.

Stefan Hajnoczi (3):
  hmp: avoid the nested event loop in handle_hmp_command()
  rcu: add drain_call_rcu_co() API
  qmp: make qmp_device_add() a coroutine

 MAINTAINERS            |  2 ++
 docs/devel/rcu.txt     | 21 ++++++++++++++++
 qapi/qdev.json         |  1 +
 include/monitor/qdev.h |  3 ++-
 include/qemu/rcu.h     |  1 +
 util/rcu-internal.h    |  8 ++++++
 monitor/hmp.c          | 28 +++++++++++----------
 monitor/qmp-cmds.c     |  2 +-
 softmmu/qdev-monitor.c | 34 +++++++++++++++++++++++---
 util/rcu-co.c          | 55 ++++++++++++++++++++++++++++++++++++++++++
 util/rcu.c             |  3 ++-
 hmp-commands.hx        |  1 +
 util/meson.build       |  2 +-
 13 files changed, 140 insertions(+), 21 deletions(-)
 create mode 100644 util/rcu-internal.h
 create mode 100644 util/rcu-co.c

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2.41.0