linux-kernel - Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

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Date:	Wed, 30 Nov 2011 11:20:56 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>, tulasidhard@...il.com
Subject: Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

> 
> There's more cases, I forgot the details of how the prio_tree stuff
> works, so please consider if its possible to also have:
> 
>   __unregister_uprobe() will observe neither old nor new
> 
> This could happen if we first munmap, __unregister_uprobe() will iterate
> past where mmap() will insert the new vma, mmap will insert the new vma,
> and __unregister_uprobe() will now not observe it.
> 

- When we iterate thro __unregister_uprobe(), we always walk from the
  root of the prio tree and not depend on the last found node. So
  __unregister_uprobe able to iterate thro the rmap without finding the
  old or the new vma would mean that the exclusive mmap_sem was dropped
  for atleast a brief period and munmap/mmap are disjoint.

Here munmap_uprobe would have reduced the count followed by the pages
being cleared.
__unregister_uprobe maintains the status quo.
mmap_uprobe would load a new set of pages without any breakpoint, since
there are no consumers, and no underlying breakpoints, it also maintains
the status quo.

> and
> 
>   __unregister_uprobe() will observe both old _and_ new
> 
> This latter could happen by favourably interleaving the prio_tree
> iteration with the munmap and mmap operations, so that we first observe
> the old vma, do the munmap, do the mmap, and then have the
> find_next_vma_info() thing find the new vma.

If __unregister_uprobe() can observe both old _and_ new, then it means
mmap has occurred. So its correct that probes are removed from
the old and new. The munmap_uprobe of the old vma wouldnt see the
breakpoint (via read_opcode) so wont decrement the count. If the
munmap_uprobe had seen the breakpoint before unregister_uprobe, then
unregister_uprobe cant decrement the count.

-- 
Thanks and Regards
Srikar

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