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Subject: Re: [rfc] "fair" rw spinlocks
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Date: Mon, 07 Dec 2009 21:38:40 +0100
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On Mon, 2009-12-07 at 19:32 +0100, Oleg Nesterov wrote:
> On 12/05, Eric W. Biederman wrote:
> >
> > Atomically sending signal to every member of a process group, is the
> > big fly in the ointment I am aware of.  Last time I looked I could
> > not see how to convert it rcu.
> 
> I am not sure, but iirc we can do this lockless (under rcu_lock).
> We need to modify pid_link to use list_entry and attach_pid() should
> add the new task to the end. Of course we need more changes, but
> (again iirc) this is not too hard.
> 
> > This is a pain because we occasionally signal a process group from
> > interrupt context.
> 
> Only send_sigio/etc does so, right?
> 
> 
> I didn't read the previous discussion yet (will try tomorrow), sorry
> if I am off-topic. But I think the nastiest problem with tasklist
> is that it protects parent/child relationship. We need per-process
> lock, but first we should change ptrace to avoid this lock somehow.
> (this is one of the goals of ptrace-utrace, but not "immediate").

Didn't Thomas and you also come up with a scheme to push most signal
processing into task context?