linux-kernel - Re: [rfc] "fair" rw spinlocks

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Date:	Mon, 7 Dec 2009 19:32:26 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks

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").

Oleg.

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