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Re: [PATCH v4 2/2] accel/tcg: Register a force_rcu notifier

From: Richard Henderson
Subject: Re: [PATCH v4 2/2] accel/tcg: Register a force_rcu notifier
Date: Wed, 10 Nov 2021 10:47:51 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 11/9/21 7:35 PM, Greg Kurz wrote:
A TCG vCPU doing a busy loop systematicaly hangs the QEMU monitor
if the user passes 'device_add' without argument. This is because
drain_cpu_all() which is called from qmp_device_add() cannot return
if readers don't exit read-side critical sections. That is typically
what busy-looping TCG vCPUs do:

int cpu_exec(CPUState *cpu)
     while (!cpu_handle_exception(cpu, &ret)) {
         // Busy loop keeps vCPU here

     return ret;

For MTTCG, have all vCPU threads register a force_rcu notifier that will
kick them out of the loop using async_run_on_cpu(). The notifier is called
with the rcu_registry_lock mutex held, using async_run_on_cpu() ensures
there are no deadlocks.

For RR, a single thread runs all vCPUs. Just register a single notifier
that kicks the current vCPU to the next one.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>

For RR:
Suggested-by: Richard Henderson <richard.henderson@linaro.org>

Fixes: 7bed89958bfb ("device_core: use drain_call_rcu in in qmp_device_add")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/650
Signed-off-by: Greg Kurz <groug@kaod.org>
  accel/tcg/tcg-accel-ops-mttcg.c | 26 ++++++++++++++++++++++++++
  accel/tcg/tcg-accel-ops-rr.c    | 10 ++++++++++
  2 files changed, 36 insertions(+)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


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