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 -- 2.41.0
