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Re: [Qemu-devel] [PATCH 13/16] cpus-common: simplify locking for start_e
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH 13/16] cpus-common: simplify locking for start_exclusive/end_exclusive |
Date: |
Mon, 26 Sep 2016 09:27:01 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.1.50.1 |
Paolo Bonzini <address@hidden> writes:
> It is not necessary to hold qemu_cpu_list_mutex throughout the
> exclusive section, because no other exclusive section can run
> while pending_cpus != 0.
>
> exclusive_idle() is called in cpu_exec_start(), and that prevents
> any CPUs created after start_exclusive() from entering cpu_exec()
> during an exclusive section.
>
> Signed-off-by: Paolo Bonzini <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
> ---
> cpus-common.c | 11 ++++++++---
> docs/tcg-exclusive.promela | 4 +++-
> include/qom/cpu.h | 4 ----
> 3 files changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/cpus-common.c b/cpus-common.c
> index 80aaf9b..429652c 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -171,8 +171,7 @@ static inline void exclusive_idle(void)
> }
>
> /* Start an exclusive operation.
> - Must only be called from outside cpu_exec, takes
> - qemu_cpu_list_lock. */
> + Must only be called from outside cpu_exec. */
> void start_exclusive(void)
> {
> CPUState *other_cpu;
> @@ -191,11 +190,17 @@ void start_exclusive(void)
> while (pending_cpus > 1) {
> qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
> }
> +
> + /* Can release mutex, no one will enter another exclusive
> + * section until end_exclusive resets pending_cpus to 0.
> + */
> + qemu_mutex_unlock(&qemu_cpu_list_lock);
> }
>
> -/* Finish an exclusive operation. Releases qemu_cpu_list_lock. */
> +/* Finish an exclusive operation. */
> void end_exclusive(void)
> {
> + qemu_mutex_lock(&qemu_cpu_list_lock);
> pending_cpus = 0;
> qemu_cond_broadcast(&exclusive_resume);
> qemu_mutex_unlock(&qemu_cpu_list_lock);
> diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
> index 9e7d9e3..a8896e5 100644
> --- a/docs/tcg-exclusive.promela
> +++ b/docs/tcg-exclusive.promela
> @@ -97,9 +97,11 @@ byte has_waiter[N_CPUS];
> do \
> :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex); \
> :: else -> break; \
> - od
> + od; \
> + MUTEX_UNLOCK(mutex);
>
> #define end_exclusive() \
> + MUTEX_LOCK(mutex); \
> pending_cpus = 0; \
> COND_BROADCAST(exclusive_resume); \
> MUTEX_UNLOCK(mutex);
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index f872614..934c07a 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -846,9 +846,6 @@ void cpu_exec_end(CPUState *cpu);
> * cpu_exec are exited immediately. CPUs that call cpu_exec_start
> * during the exclusive section go to sleep until this CPU calls
> * end_exclusive.
> - *
> - * Returns with the CPU list lock taken (which nests outside all
> - * other locks except the BQL).
> */
> void start_exclusive(void);
>
> @@ -856,7 +853,6 @@ void start_exclusive(void);
> * end_exclusive:
> *
> * Concludes an exclusive execution section started by start_exclusive.
> - * Releases the CPU list lock.
> */
> void end_exclusive(void);
--
Alex Bennée
- [Qemu-devel] [PATCH 08/16] cpus-common: fix uninitialized variable use in run_on_cpu, (continued)
- [Qemu-devel] [PATCH 08/16] cpus-common: fix uninitialized variable use in run_on_cpu, Paolo Bonzini, 2016/09/23
- [Qemu-devel] [PATCH 10/16] docs: include formal model for TCG exclusive sections, Paolo Bonzini, 2016/09/23
- [Qemu-devel] [PATCH 09/16] cpus-common: move exclusive work infrastructure from linux-user, Paolo Bonzini, 2016/09/23
- [Qemu-devel] [PATCH 11/16] cpus-common: always defer async_run_on_cpu work items, Paolo Bonzini, 2016/09/23
- [Qemu-devel] [PATCH 13/16] cpus-common: simplify locking for start_exclusive/end_exclusive, Paolo Bonzini, 2016/09/23
- [Qemu-devel] [PATCH 15/16] tcg: Make tb_flush() thread safe, Paolo Bonzini, 2016/09/23
[Qemu-devel] [PATCH 14/16] cpus-common: Introduce async_safe_run_on_cpu(), Paolo Bonzini, 2016/09/23
[Qemu-devel] [PATCH 16/16] cpus-common: lock-free fast path for cpu_exec_start/end, Paolo Bonzini, 2016/09/23