[v2,0/2] accel/tcg: Fix monitor deadlock

Message

Greg Kurz Oct. 19, 2021, 5:56 a.m. UTC

Commit 7bed89958bfb ("device_core: use drain_call_rcu in in qmp_device_add")
introduced a regression in QEMU 6.0 : passing device_add without argument
hangs the monitor. This was reported against qemu-system-mips64 with TGC,
but I could consistently reproduce it with other targets (x86 and ppc64).

See https://gitlab.com/qemu-project/qemu/-/issues/650 for details.

The problem is that an emulated busy-looping vCPU can stay forever in
its RCU read-side critical section and prevent drain_call_rcu() to return.
This series fixes the issue by letting RCU kick vCPUs out of the read-side
critical section when drain_call_rcu() is in progress. This is achieved
through notifiers, as suggested by Paolo Bonzini.

v2:
- moved notifier list to RCU reader data
- separate API for notifier registration
- CPUState passed as an opaque pointer

Greg Kurz (2):
  rcu: Introduce force_rcu notifier
  accel/tcg: Register a force_rcu notifier

 accel/tcg/tcg-accel-ops-mttcg.c |  3 +++
 accel/tcg/tcg-accel-ops-rr.c    |  3 +++
 accel/tcg/tcg-accel-ops.c       | 15 +++++++++++++++
 accel/tcg/tcg-accel-ops.h       |  2 ++
 include/qemu/rcu.h              | 16 ++++++++++++++++
 util/rcu.c                      | 22 +++++++++++++++++++++-
 6 files changed, 60 insertions(+), 1 deletion(-)