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Date: Wed, 20 Feb 2013 23:37:03 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Pavel Emelyanov <xemul@parallels.com>
Cc: mtk.manpages@gmail.com, Andrey Vagin <avagin@openvz.org>,
        linux-kernel@vger.kernel.org, criu@openvz.org,
        linux-api@vger.kernel.org, Roland McGrath <roland@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>, Dave Jones <davej@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] ptrace: add ability to retrieve signals without
	removing them from a queue
Message-ID: <20130220223703.GB16194@redhat.com>
References: <1360768595-31943-1-git-send-email-avagin@openvz.org> <CAKgNAkicAGEkvNFnSso__wK6QKao8jyDu5yuRmeyUOeaXLGChw@mail.gmail.com> <5123BC20.9040405@parallels.com> <20130219193424.GA9606@redhat.com> <5123D6D6.7010904@parallels.com>
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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.