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Date: Mon, 7 Dec 2009 19:32:26 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       Christoph Hellwig <hch@infradead.org>, Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks
Message-ID: <20091207183226.GA20139@redhat.com>
References: <20091130100041.GA29610@infradead.org> <alpine.LFD.2.00.0911300722370.2872@localhost.localdomain> <20091130174638.GA9782@elte.hu> <alpine.LFD.2.00.0911302206370.24119@localhost.localdomain> <1259616429.26472.499.camel@laptop> <alpine.LFD.2.00.0911302300550.24119@localhost.localdomain> <alpine.LFD.2.00.0911301409560.2872@localhost.localdomain> <alpine.LFD.2.00.0911302328350.24119@localhost.localdomain> <alpine.LFD.2.00.0911301443110.2872@localhost.localdomain> <m1y6lg7q3n.fsf@fess.ebiederm.org>
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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.