Re: [PATCH] RT: Checks for cmpxchg in get_task_struct_rcu()

[Nick Piggin <nickpiggin@yahoo.com.au>]

Daniel Walker wrote:
> Adds a check for cmpxchg in get_task_struct_rcu(), and implements the
> case when it doesn't exist.

I believe this is racy.

> Signed-Off-By: Daniel Walker <dwalker@mvista.com>
>
> Index: linux-2.6.13/include/linux/sched.h
> ===================================================================
> --- linux-2.6.13.orig/include/linux/sched.h
> +++ linux-2.6.13/include/linux/sched.h
> @@ -1026,13 +1026,21 @@ static inline int get_task_struct_rcu(st
>  {
>  	int oldusage;
>  
> +#ifdef __HAVE_ARCH_CMPXCHG
>  	do {
>  		oldusage = atomic_read(&t->usage);
>  		if (oldusage == 0) {
>  			return 0;
>  		}
>  	} while (cmpxchg(&t->usage.counter,
> -		 oldusage, oldusage + 1) != oldusage);
> +				oldusage, oldusage + 1) != oldusage);
> +#else
> +	oldusage = atomic_read(&t->usage);
> +	if (oldusage == 0) {
> +		return 0;
> +	}

What if t->usage becomes 0 here?

> +	atomic_inc(&t->usage);
> +#endif
>  	return 1;
>  }
>  

You need my atomic_cmpxchg patches that provide an atomic_cmpxchg
(and atomic_inc_not_zero) for all architectures.

--
SUSE Labs, Novell Inc.

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