Re: [Qemu-devel] [PATCH v3 1/5] rcu: Add automatically released rcu_read_lock variants

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/09/19 12:25, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> RCU_READ_LOCK_GUARD() takes the rcu_read_lock and then uses glib's
> g_auto infrastructure (and thus whatever the compiler's hooks are) to
> release it on all exits of the block.
> 
> WITH_RCU_READ_LOCK_GUARD() is similar but is used as a wrapper for the
> lock, i.e.:
> 
>    WITH_RCU_READ_LOCK_GUARD() {
>        stuff under lock
>    }
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  docs/devel/rcu.txt | 16 ++++++++++++++++
>  include/qemu/rcu.h | 25 +++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/docs/devel/rcu.txt b/docs/devel/rcu.txt
> index c84e7f42b2..d83fed2f79 100644
> --- a/docs/devel/rcu.txt
> +++ b/docs/devel/rcu.txt
> @@ -187,6 +187,22 @@ The following APIs must be used before RCU is used in a thread:
>  Note that these APIs are relatively heavyweight, and should _not_ be
>  nested.
>  
> +Convenience macros
> +==================
> +
> +Two macros are provided that automatically release the read lock at the
> +end of the scope.
> +
> +      RCU_READ_LOCK_GUARD()
> +
> +         Takes the lock and will release it at the end of the block it's
> +         used in.
> +
> +      WITH_RCU_READ_LOCK_GUARD()  { code }
> +
> +         Is used at the head of a block to protect the code within the block.
> +
> +Note that 'goto'ing out of the guarded block will also drop the lock.
>  
>  DIFFERENCES WITH LINUX
>  ======================
> diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> index 22876d1428..3a8d4cf28b 100644
> --- a/include/qemu/rcu.h
> +++ b/include/qemu/rcu.h
> @@ -154,6 +154,31 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
>        }),                                                                \
>        (RCUCBFunc *)g_free);
>  
> +typedef void RCUReadAuto;
> +static inline RCUReadAuto *rcu_read_auto_lock(void)
> +{
> +    rcu_read_lock();
> +    /* Anything non-NULL causes the cleanup function to be called */
> +    return (void *)(uintptr_t)0x1;
> +}
> +
> +static inline void rcu_read_auto_unlock(RCUReadAuto *r)
> +{
> +    rcu_read_unlock();
> +}
> +
> +G_DEFINE_AUTOPTR_CLEANUP_FUNC(RCUReadAuto, rcu_read_auto_unlock)
> +
> +#define WITH_RCU_READ_LOCK_GUARD() \
> +    WITH_RCU_READ_LOCK_GUARD_(_rcu_read_auto##__COUNTER__)
> +
> +#define WITH_RCU_READ_LOCK_GUARD_(var) \
> +    for (g_autoptr(RCUReadAuto) var = rcu_read_auto_lock(); \
> +        (var); rcu_read_auto_unlock(var), (var) = NULL)
> +
> +#define RCU_READ_LOCK_GUARD() \
> +    g_autoptr(RCUReadAuto) _rcu_read_auto = rcu_read_auto_lock()
> +
>  #ifdef __cplusplus
>  }
>  #endif
> 

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