Re: [Qemu-devel] [PATCH] rcu: do not create thread in pthread_atfork callback

[Peter Maydell <peter.maydell@linaro.org>]

On 31 March 2015 at 12:01, Paolo Bonzini <pbonzini@redhat.com> wrote:
> If QEMU forks after the CPU threads have been created, qemu_mutex_lock_iothread
> will not be able to do qemu_cpu_kick_thread.  There is no solution other than
> assuming that forks after the CPU threads have been created will end up in an
> exec.

This assumption is false for linux-user mode...
(though in that case we don't have an iothread).

> diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> index 506ab58..7df1e86 100644
> --- a/include/qemu/rcu.h
> +++ b/include/qemu/rcu.h
> @@ -117,6 +117,7 @@ extern void synchronize_rcu(void);
>   */
>  extern void rcu_register_thread(void);
>  extern void rcu_unregister_thread(void);
> +extern void rcu_after_fork(void);
>
>  struct rcu_head;
>  typedef void RCUCBFunc(struct rcu_head *head);
> diff --git a/os-posix.c b/os-posix.c
> index ba091f1..e4da406 100644
> --- a/os-posix.c
> +++ b/os-posix.c
> @@ -39,6 +39,7 @@
>  #include "sysemu/sysemu.h"
>  #include "net/slirp.h"
>  #include "qemu-options.h"
> +#include "qemu/rcu.h"
>
>  #ifdef CONFIG_LINUX
>  #include <sys/prctl.h>
> @@ -247,6 +248,7 @@ void os_daemonize(void)
>          signal(SIGTSTP, SIG_IGN);
>          signal(SIGTTOU, SIG_IGN);
>          signal(SIGTTIN, SIG_IGN);
> +        rcu_after_fork();
>      }
>  }
>
> diff --git a/util/rcu.c b/util/rcu.c
> index 27802a4..7270151 100644
> --- a/util/rcu.c
> +++ b/util/rcu.c
> @@ -311,19 +311,18 @@ static void rcu_init_unlock(void)
>  {
>      qemu_mutex_unlock(&rcu_gp_lock);
>  }
> +#endif
>
> -static void rcu_init_child(void)
> +void rcu_after_fork(void)
>  {
> -    qemu_mutex_unlock(&rcu_gp_lock);
>      memset(&registry, 0, sizeof(registry));
>      rcu_init_complete();
>  }
> -#endif
>
>  static void __attribute__((__constructor__)) rcu_init(void)
>  {
>  #ifdef CONFIG_POSIX
> -    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
> +    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_unlock);
>  #endif
>      rcu_init_complete();
>  }

So this is changing the linux-user behaviour so we no
longer do any init after fork; is that a problem?

-- PMM