linux-kernel - Re: [patch 2/13] signal/timer/event fds v6

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Date:	Tue, 20 Mar 2007 02:28:42 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 2/13] signal/timer/event fds v6 - signalfd core ...

On 03/20, Oleg Nesterov wrote:
>
> On 03/19, Davide Libenzi wrote:
> >
> > I'd need a get_task_struct in any case in order to safely call 
> > unlock_task_sighand(). At that point I'd prefer to just pass through the 
> > struct pid*. I'll be posting the new version for review as soon as I 
> > complete a few tests ...
> 
> If signalfd_get_sighand()->lock_task_sighand() succeeds, it is safe to
> dereference ctx->tsk. The task can't be freed and ctx->tsk can't be cleared
> while we are holding siglock.
> 
> However, I was wrong, we still need a re-check after lock_task_sighand().
> We should check ctx->tsk != NULL.

IOW, we can (afaics) do

	static struct sighand_struct *signalfd_get_sighand(struct signalfd_ctx *ctx,
							   unsigned long *flags)
	{
		struct sighand_struct *sighand = NULL;
		struct tsak_struct *tsk;

		rcu_read_lock();
		tsk = rcu_dereference(ctx->tsk);  // not needed, just a documentation
		if (tsk != NULL)
			sighand = lock_task_sighand(tsk, flags);
		rcu_read_unlock();

		if (sighand && !ctx->tsk)) {
			unlock_task_sighand(tsk, flags);
			sighand = NULL;
		}

		return sighand;
	}

If signalfd_get_sighand() succeeds, ctx->tsk is pinned.

Oleg.

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