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

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Date:	Thu, 21 Feb 2013 14:30:26 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Oleg Nesterov <oleg@...hat.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 02/21/2013 02:37 AM, Oleg Nesterov wrote:
> 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.

Or allocate intermediate buffer putting the siginfo's there.

> Oleg.

Thanks,
Pavel
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