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SubjectRe: [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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