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 19:55:07 +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.

* Peter Zijlstra <peterz@...radead.org> [2011-11-24 15:13:37]:

> 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.
> 

In the success case, install_breakpoint itself does the increment.
We cant allow install_breakpoint to increment in EEXIST case always
because doing that in register_uprobe context would increment which is
wrong.

> 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.

Okay .. 

-- 
Thanks and Regards
Srikar

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