linux-kernel - Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20] 8: uprobes: mmap and fork hooks.

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Date:	Wed, 26 Jan 2011 14:33:46 +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>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20]  8: uprobes: mmap and fork
 hooks.

> On Thu, 2010-12-16 at 15:28 +0530, Srikar Dronamraju wrote:
> > +void uprobe_mmap(struct vm_area_struct *vma)
> > +{
> > +       struct list_head tmp_list;
> > +       struct uprobe *uprobe, *u;
> > +       struct mm_struct *mm;
> > +       struct inode *inode;
> > +
> > +       if (!valid_vma(vma))
> > +               return;
> > +
> > +       INIT_LIST_HEAD(&tmp_list);
> > +
> > +       /*
> > +        * The vma was just allocated and this routine gets called
> > +        * while holding write lock for mmap_sem.  Function called
> > +        * in context of a thread that has a reference to mm.
> > +        * Hence no need to take a reference to mm
> > +        */
> > +       mm = vma->vm_mm;
> > +       up_write(&mm->mmap_sem);
> 
> Are you very very sure its a good thing to simply drop the mmap_sem
> here? Also, why?
> 

I actually dont like to release the write_lock and then reacquire it.
write_opcode, which is called thro install_uprobe, i.e to insert the
actual breakpoint instruction takes a read lock on the mmap_sem.
Hence uprobe_mmap gets called in context with write lock on mmap_sem
held, I had to release it before calling install_uprobe.

Another solution, I thought of was to pass a context to write_opcode to
say that map-sem is already acquired by us. But I am not sure that
idea is good enuf. 

> > +       mutex_lock(&uprobes_mutex);
> > +
> > +       inode = vma->vm_file->f_mapping->host;
> 
> Since you just dropped the mmap_sem, what's keeping that vma from going
> away?
> 

How about dropping the mmap_sem after add_to_temp_list and cachng the
vma->vm_start value before calling add_to_temp_list?

Or if you have better ideas, then that would be great.

> > +       add_to_temp_list(vma, inode, &tmp_list);
> > +
> > +       list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
> > +               mm->uprobes_vaddr = vma->vm_start + uprobe->offset;
> > +               install_uprobe(mm, uprobe);
> > +               list_del(&uprobe->pending_list);
> > +       }
> > +       mutex_unlock(&uprobes_mutex);
> > +       down_write(&mm->mmap_sem);
> > +} 
> 
> 
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