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

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Date:	Thu, 24 Nov 2011 15:13:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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.

On Thu, 2011-11-24 at 19:17 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2011-11-23 19:10:12]:
> 
> > On Fri, 2011-11-18 at 16:37 +0530, Srikar Dronamraju wrote:
> > > +                       ret = install_breakpoint(vma->vm_mm, uprobe);
> > > +                       if (ret == -EEXIST) {
> > > +                               atomic_inc(&vma->vm_mm->mm_uprobes_count);
> > > +                               ret = 0;
> > > +                       } 
> > 
> > Aren't you double counting that probe position here? The one that raced
> > you to inserting it will also have incremented that counter, no?
> > 
> 
> No we arent.
> Because register_uprobe can never race with mmap_uprobe and register
> before mmap_uprobe registers .(Once we start mmap_region,
> register_uprobe waits for the read_lock of mmap_sem.)

Still doesn't make any sense. Since you don't increment on success, one
has to assume install_breakpoint() will cause an increment. Therefore,
when we encounter -EEXIST we'll already have accounted for this
mm,inode,offset combination.

But I'll have another look at it, maybe I'm missing something
obvious :-)

> And we badly need this for mmap_uprobe case.  Because when we do mremap,
> or vma_adjust(), we do a munmap_uprobe() followed by mmap_uprobe() which
> would have decremented the count but not removed it. So when we do a
> mmap_uprobe, we need to increment the count. 

Well I see why the count needs to be correct, that's not the issue.
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