Re: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks.

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-06-24 at 07:36 +0530, Srikar Dronamraju wrote:
> > 
> > so I am thinking of a solution that includes most of your ideas along
> > with using i_mmap_mutex in mmap_uprobe path.
> > 
> 
> Addressing Peter's comments given on irc wrt i_mmap_mutex.
> 

> void _unregister_uprobe(...)
> {
> 	if (!del_consumer(...)) {	// includes tree removal on last consumer
> 		return;
> 	}
> 	if (uprobe->consumers)
> 		return;
> 
> 	mutex_lock(&mapping->i_mmap_mutex);	//sync with mmap.
> 	vma_prio_tree_foreach() {
> 		// create list
> 	}
> 
> 	mutex_unlock(&mapping->i_mmap_mutex);
> 
> 	list_for_each_entry_safe() {
> 		// remove from list
> 		down_read(&mm->mmap_sem);
> 		remove_breakpoint();	// unconditional, if it wasn't there
> 		up_read(&mm->mmap_sem);
> 	}
> 
> 	mutex_lock(&mapping->i_mmap_mutex);
> 	delete_uprobe(uprobe);
> 	mutex_unlock(&mapping->i_mmap_mutex);
> 
> 	inode->uprobes_count--;
> 	mutex_unlock(&inode->i_mutex);

Right, so this lonesome unlock got me puzzled for a while, I always find
it best not to do asymmetric locking like this, keep the lock and unlock
in the same function.

> }
> 
> int register_uprobe(...)
> {
> 	uprobe = alloc_uprobe(...);	// find or insert in tree
> 
> 	mutex_lock(&inode->i_mutex);	// sync with register/unregister
> 	if (uprobe->consumers) {
> 		add_consumer();
> 		goto put_unlock;
> 	}
> 	add_consumer();
> 	inode->uprobes_count++;
> 	mutex_lock(&mapping->i_mmap_mutex);	//sync with mmap.
> 	vma_prio_tree_foreach(..) {
> 		// get mm ref, add to list blah blah
> 	}
> 
> 	mutex_unlock(&mapping->i_mmap_mutex);
> 	list_for_each_entry_safe() {
> 		if (ret) {
> 			// del from list etc..
> 			//
> 			continue;
> 		}
> 		down_read(mm->mmap_sem);
> 		ret = install_breakpoint();
> 		up_read(..);
> 		// del from list etc..
> 		//
> 		if (ret && (ret == -ESRCH || ret == -EEXIST))
> 			ret = 0;
> 	}
> 
> 	if (ret)
> 		_unregister_uprobe();
> 
>       put_unlock:
> 	mutex_unlock(&inode->i_mutex);

You see, now this is a double unlock

> 	put_uprobe(uprobe);
> 	return ret;
> }
> 
> void unregister_uprobe(...)
> {
> 	mutex_lock(&inode->i_mutex);	// sync with register/unregister
> 	uprobe = find_uprobe();	// ref++
> 	_unregister_uprobe();
> 	mutex_unlock(&inode->i_mutex);

idem

> 	put_uprobe(uprobe);
> }
> 
> int mmap_uprobe(struct vm_area_struct *vma)
> {
> 	struct list_head tmp_list;
> 	struct uprobe *uprobe, *u;
> 	struct mm_struct *mm;
> 	struct inode *inode;
> 	int ret = 0;
> 
> 	if (!valid_vma(vma))
> 		return ret;	/* Bail-out */
> 
> 	mm = vma->vm_mm;
> 	inode = vma->vm_file->f_mapping->host;
> 	if (inode->uprobes_count)
> 		return ret;
> 	__iget(inode);
> 
> 	INIT_LIST_HEAD(&tmp_list);
> 
> 	mutex_lock(&mapping->i_mmap_mutex);
> 	add_to_temp_list(vma, inode, &tmp_list);
> 	list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
> 		loff_t vaddr;
> 
> 		list_del(&uprobe->pending_list);
> 		if (ret)
> 			continue;
> 
> 		vaddr = vma->vm_start + uprobe->offset;
> 		vaddr -= vma->vm_pgoff << PAGE_SHIFT;
> 		ret = install_breakpoint(mm, uprobe, vaddr);

Right, so this is the problem, you cannot do allocations under
i_mmap_mutex, however I think you can under i_mutex.

> 		if (ret && (ret == -ESRCH || ret == -EEXIST))
> 			ret = 0;
> 	}
> 
> 	mutex_unlock(&mapping->i_mmap_mutex);
> 	iput(inode);
> 	return ret;
> }
> 
> int munmap_uprobe(struct vm_area_struct *vma)
> {
> 	struct list_head tmp_list;
> 	struct uprobe *uprobe, *u;
> 	struct mm_struct *mm;
> 	struct inode *inode;
> 	int ret = 0;
> 
> 	if (!valid_vma(vma))
> 		return ret;	/* Bail-out */
> 
> 	mm = vma->vm_mm;
> 	inode = vma->vm_file->f_mapping->host;
> 	if (inode->uprobes_count)
> 		return ret;

Should that be !->uprobes_count?

> //      walk thro RB tree and decrement mm->uprobes_count
> 	walk_rbtree_and_dec_uprobes_count();	//hold treelock.
> 
> 	return ret;
> }

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