linux-kernel - Re: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes

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Message-Id: <20100722192446.E1258400B8@magilla.sf.frob.com>
Date:	Thu, 22 Jul 2010 12:24:46 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>, andi.kleen@...el.com,
	stable@...nel.org
Subject: Re: [PATCH] Don't apply for write lock on tasklist_lock if parent
	doesn't ptrace other processes

> So, this list_empty() check needs taskslit at least for reading. But, we
> are going to take it for writing right after exit_ptrace() returns, afaics
> we can add this fastpatch check for free.

That looks good to me, but it could use some more scare comments.

>  /*
> - * Detach all tasks we were using ptrace on.
> + * Detach all tasks we were using ptrace on. Called with tasklist held.

 * Called with tasklist held for writing, and returns with it held too.
 * But note it can release and reacquire the lock.

> +	write_lock_irq(&tasklist_lock);
> +
	/*
	 * Note that exit_ptrace() might drop tasklist_lock and reacquire it.
	 */
>  	exit_ptrace(father);
>  
> -	write_lock_irq(&tasklist_lock);
>  	reaper = find_new_reaper(father);


Thanks,
Roland
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