linux-kernel - Re: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks.

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Message-ID: <20110719065350.GB1210@linux.vnet.ibm.com>
Date:	Tue, 19 Jul 2011 12:23:50 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 7/22]  7: uprobes: mmap and fork hooks.

* Peter Zijlstra <peterz@...radead.org> [2011-07-18 16:31:16]:

> On Mon, 2011-07-18 at 14:50 +0530, Srikar Dronamraju wrote:
> >  *  - Introduce uprobes_list and uprobes_vaddr in vm_area_struct.
> >  *    uprobes_list is a node in the temp list of vmas while
> >  *    registering/unregistering uprobes. uprobes_vaddr caches the vaddr to
> >  *    insert/remove the breakpoint.
> >  *
> >  *  - Introduce srcu to synchronize vma deletion with walking the list of
> >  *    vma in register/unregister_uprobe.
> 
> I don't think you can sell this, that'll make munmap() horridly slow.

Okay, 

How about using a counter and a wq in each vma.
Based on the counter, I can wait in the munmap() and since this is per
vma, this should be faster than srcu.

Counter would be incremented when we do a vma-rmap walk. 
decremented when after insertion/deletion.
read in munmap().

> 
> >  *  - Introduce uprobes_mmap_mutex to synchronize uprobe deletion and
> >  *    mmap_uprobe(). 
> 
> Yes, that'll work I think.

Here another possibility is to have a per uprobe mutex, that way we
would not have to serialize mmap_uprobe. But this optimization can be
done later too.

-- 
Thanks and Regards
Srikar
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