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

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Message-ID: <Pine.LNX.4.64.0703191530270.2101@alien.or.mcafeemobile.com>
Date:	Mon, 19 Mar 2007 15:33:25 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Oleg Nesterov <oleg@...sign.ru>
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 Mon, 19 Mar 2007, Oleg Nesterov wrote:

> 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).

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 ...



- Davide


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