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

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Date:	Mon, 19 Mar 2007 23:36:15 +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 03/19, Davide Libenzi wrote:
>
> On Mon, 19 Mar 2007, Eric W. Biederman wrote:
> 
> > > +struct signalfd_ctx {
> > > +	struct list_head lnk;
> > > +	wait_queue_head_t wqh;
> > > +	sigset_t sigmask;
> > > +	struct task_struct *tsk;
> > > +};
> > 
> > I think you want to use a struct pid *pid instead of a pointer to the
> > task struct here.  It is slightly less efficient (one more
> > dereference) but it means that we won't pin the task struct in memory
> > indefinitely.  Pinning the task_struct like this makes for a very
> > interesting way to get around the limits on the number of processes a
> > user can have.
> 
> Hmm, when the task is detached from the sighand, we get a notify, so I 
> could do a put from there. This would avoid the extra de-reference. I need 
> to verify locking though ...

In that case (if I understand you correctly) we don't need {get,put}_task_struct()
at all.

signalfd_deliver(-1) sets ctx->tsk = NULL, signalfd_get_sighand() reads ->tsk
under rcu_read_lock(). The code becomes even simpler, we don't need to check
list_empty(&ctx->lnk).

Oleg.

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