linux-kernel - Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu

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Date:	Sat, 14 Apr 2012 15:16:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

On Fri, 2012-04-06 at 00:20 +0200, Oleg Nesterov wrote:
> Hello.
> 
> Not for inclusion yet, only for the early review.
> 
> I didn't even try to test these changes, and I am not expert
> in this area. And even _if_ this code is correct, I need to
> re-split these changes anyway, update the changelogs, etc.
> 
> Questions:
> 
> 	- does it make sense?

Maybe, upside is reclaiming that int from task_struct, downside is that
down_write :/ It would be very good not to have to do that. Nor do I
really see how that works.

> 	- can it work or I missed something "in general" ?

So we insert in the rb-tree before we take mmap_sem, this means we can
hit a non-uprobe int3 and still find a uprobe there, no?

> Why:
> 
> 	- It would be nice to remove a member from task_struct.
> 
> 	- Afaics, the usage of uprobes_srcu does not look right,
> 	  at least in theory, see 6/6.
> 
> 	  The comment above delete_uprobe() says:
> 
> 	  	The current unregistering thread waits till all
> 	  	other threads have hit a breakpoint, to acquire
> 	  	the uprobes_treelock before the uprobe is removed
> 	  	from the rbtree.
> 
> 	  but synchronize_srcu() can only help if a thread which
> 	  have hit the breakpoint has already called srcu_read_lock().
> 	  It can't synchronize with read_lock "in future", and there
> 	  is a small window.
> 
> 	  We could probably add another synchronize_sched() before
> 	  synchronize_srcu(), but this doesn't look very nice and

Right, I think that all was written with the assumption that sync_srcu
implied a sync_rcu, which of course we've recently wrecked.


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