linux-kernel - Re: [PATCH] ptrace: add ability to retrieve signals without removing them from a queue

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Date:	Wed, 20 Feb 2013 23:37:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	mtk.manpages@...il.com, Andrey Vagin <avagin@...nvz.org>,
	linux-kernel@...r.kernel.org, criu@...nvz.org,
	linux-api@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] ptrace: add ability to retrieve signals without
	removing them from a queue

On 02/19, Pavel Emelyanov wrote:
>
> On 02/19/2013 11:34 PM, Oleg Nesterov wrote:
> > But, given that every PEEK does list_for_each() until it finds the
> > necessary sequence number, I am wondering how this O(n**2) will work
> > if you want to dump 126065 signals ;)
>
> Isn't it the great reason for making the addr point to a structure, that
> would look like
>
> struct siginfo_peek_arg {
> 	unsigned flags; /* all bits but 0th, that selects between private/shared
> 			   queues, should be zero */
> 	unsigned int off; /* from which siginfo to start */
> 	unsigned int nr; /* how may siginfos to take */
> };

I am fine either way, to me everything looks better than signalfd
hacks.

But if you meant "avoid n^2", this won't help? You can't do
copy_siginfo_to_user() under ->siglock, so you need to restart
list_for_each() anyway.

Oleg.

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