linux-kernel - Re: [PATCH -v2] futex: fix reference leak

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Date:	Wed, 11 Feb 2009 13:28:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Darren Hart <dvhltc@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH -v2] futex: fix reference leak


* Peter Zijlstra <peterz@...radead.org> wrote:

> +	if (!unqueue_me(&q)) {
> +		ret = 0;
> +		goto out_put_key;
> +	}
> +	if (rem) {
> +		ret = -ETIMEDOUT;
> +		goto out_put_key;
> +	}

hm, we generally prefer to write such things as:

	ret = 0;
	if (!unqueue_me(&q))
		goto out_put_key;

	ret = -ETIMEDOUT;
	if (rem)
		goto out_put_key;

Also, while at it, the flow of control looks weird in other places too:

        if (!abs_time)
                return -ERESTARTSYS;
        else {
                struct restart_block *restart;
                restart = &current_thread_info()->restart_block;
                restart->fn = futex_wait_restart;
                restart->futex.uaddr = (u32 *)uaddr;

Shouldnt it be something like:

        if (!abs_time)
                return -ERESTARTSYS;

        restart = &current_thread_info()->restart_block;
        restart->fn = futex_wait_restart;
        restart->futex.uaddr = (u32 *)uaddr;

(with the variable definition moving up to local variables.)

and this:

> +	if (!abs_time) {
> +		ret = -ERESTARTSYS;
> +		goto out_put_key;
> +	} else {
>  		struct restart_block *restart;
>  		restart = &current_thread_info()->restart_block;
>  		restart->fn = futex_wait_restart;
> @@ -1309,11 +1314,13 @@ retry:
>  			restart->futex.flags |= FLAGS_SHARED;
>  		if (clockrt)
>  			restart->futex.flags |= FLAGS_CLOCKRT;
> -		return -ERESTART_RESTARTBLOCK;
> +		ret = -ERESTART_RESTARTBLOCK;
> +		goto out_put_key;
>  	}
>  
>  out_unlock_put_key:
>  	queue_unlock(&q, hb);
> +out_put_key:
>  	put_futex_key(fshared, &q.key);

and this looks weird too, we jump-goto over the queue_unlock() in essence.

A proper flow would be to rename the error labels as err_unlock_*[etc], move
them out of line, let them jump back to the normal labels - and let the
tail section of the code above fall through.

	Ingo
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