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Date: Wed, 26 Jan 2011 14:33:46 +0530
From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Linux-mm <linux-mm@kvack.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Christoph Hellwig <hch@infradead.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        SystemTap <systemtap@sources.redhat.com>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Andi Kleen <andi@firstfloor.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20]  8: uprobes: mmap and fork
 hooks.
Message-ID: <20110126090346.GH19725@linux.vnet.ibm.com>
Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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 <1295957739.28776.717.camel@laptop>
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> 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);
> > +} 
> 
>