Re: [Qemu-devel] [PATCH 1/9] main-loop: use qemu_mutex_lock_iothread consistently

[Frederic Konrad <fred.konrad@greensocs.com>]

On 18/06/2015 18:47, Paolo Bonzini wrote:
> The next patch will require the BQL to be always taken with
> qemu_mutex_lock_iothread(), while right now this isn't the case.
>
> Outside TCG mode this is not a problem.  In TCG mode, we need to be
> careful and avoid the "prod out of compiled code" step if already
> in a VCPU thread.  This is easily done with a check on current_cpu,
> i.e. qemu_in_vcpu_thread().
>
> Hopefully, multithreaded TCG will get rid of the whole logic to kick
> VCPUs whenever an I/O event occurs!
Hopefully :), this means dropping the iothread mutex as soon as possible and
removing the iothread_requesting_mutex I guess..

Fred

>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   cpus.c | 13 ++++++++-----
>   1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index de6469f..2e807f9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -924,7 +924,7 @@ static void *qemu_kvm_cpu_thread_fn(void *arg)
>       CPUState *cpu = arg;
>       int r;
>   
> -    qemu_mutex_lock(&qemu_global_mutex);
> +    qemu_mutex_lock_iothread();
>       qemu_thread_get_self(cpu->thread);
>       cpu->thread_id = qemu_get_thread_id();
>       cpu->can_do_io = 1;
> @@ -1004,10 +1004,10 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>   {
>       CPUState *cpu = arg;
>   
> +    qemu_mutex_lock_iothread();
>       qemu_tcg_init_cpu_signals();
>       qemu_thread_get_self(cpu->thread);
>   
> -    qemu_mutex_lock(&qemu_global_mutex);
>       CPU_FOREACH(cpu) {
>           cpu->thread_id = qemu_get_thread_id();
>           cpu->created = true;
> @@ -1118,7 +1118,11 @@ bool qemu_in_vcpu_thread(void)
>   
>   void qemu_mutex_lock_iothread(void)
>   {
>       atomic_inc(&iothread_requesting_mutex);
> -    if (!tcg_enabled() || !first_cpu || !first_cpu->thread) {
> +    /* In the simple case there is no need to bump the VCPU thread out of
> +     * TCG code execution.
> +     */
> +    if (!tcg_enabled() || qemu_in_vcpu_thread() ||
> +        !first_cpu || !first_cpu->thread) {
>           qemu_mutex_lock(&qemu_global_mutex);
>           atomic_dec(&iothread_requesting_mutex);
>       } else {