Re: [PULL v2 09/36] accel/tcg: Unregister the RCU before exiting RR thread

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 10/7/25 16:33, Peter Maydell wrote:
> On Fri, 4 Jul 2025 at 14:25, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>>
>> Although unreachable, still unregister the RCU before exiting
>> the thread, as documented in "qemu/rcu.h":
>>
>>   /*
>>    * Important !
>>    *
>>    * Each thread containing read-side critical sections must be registered
>>    * with rcu_register_thread() before calling rcu_read_lock().
>>    * rcu_unregister_thread() should be called before the thread exits.
>>    */
>>
>> Unregister the RCU to be on par with what is done for other
>> accelerators.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>> Acked-by: Richard Henderson <richard.henderson@linaro.org>
>> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
>> Message-Id: <20250702185332.43650-66-philmd@linaro.org>
>> ---
>>   accel/tcg/tcg-accel-ops-rr.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/accel/tcg/tcg-accel-ops-rr.c b/accel/tcg/tcg-accel-ops-rr.c
>> index 6eec5c9eee9..a578698d071 100644
>> --- a/accel/tcg/tcg-accel-ops-rr.c
>> +++ b/accel/tcg/tcg-accel-ops-rr.c
>> @@ -302,6 +302,8 @@ static void *rr_cpu_thread_fn(void *arg)
>>           rr_deal_with_unplugged_cpus();
>>       }
>>
>> +    rcu_unregister_thread();
>> +
>>       g_assert_not_reached();
>>   }
> 
> This has reintroduced CID 1547782 (unreachable code).
> 
> We can't get to this point, so why are we trying to call a
> function here ? This is not a place where the thread can exit.

The goal is to unify accelerators vcpu thread logic and
eventually remove AccelOpsClass::create_vcpu_thread(), superseded
by AccelOpsClass::cpu_thread_routine(). I couldn't finish RR for
10.1 but squeezed this patch in. Let's revert it, and I'll
re-commit it later.