Re: [PATCH] swapoff tmpfs radix_tree: remember to rcu_read_unlock

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Feb 2014 18:45:07 -0800 (PST) Hugh Dickins <hughd@google.com> wrote:

> Running fsx on tmpfs with concurrent memhog-swapoff-swapon, lots of
> 
> BUG: sleeping function called from invalid context at kernel/fork.c:606
> in_atomic(): 0, irqs_disabled(): 0, pid: 1394, name: swapoff
> 1 lock held by swapoff/1394:
>  #0:  (rcu_read_lock){.+.+.+}, at: [<ffffffff812520a1>] radix_tree_locate_item+0x1f/0x2b6
> followed by
> ================================================
> [ BUG: lock held when returning to user space! ]
> 3.14.0-rc1 #3 Not tainted
> ------------------------------------------------
> swapoff/1394 is leaving the kernel with locks still held!
> 1 lock held by swapoff/1394:
>  #0:  (rcu_read_lock){.+.+.+}, at: [<ffffffff812520a1>] radix_tree_locate_item+0x1f/0x2b6
> after which the system recovered nicely.
> 
> Whoops, I long ago forgot the rcu_read_unlock() on one unlikely branch.
> 
> Fixes: e504f3fdd63d ("tmpfs radix_tree: locate_item to speed up swapoff")

huh.  Venerable.  I'm surprised that such an obvious blooper wasn't
spotted at review.  Why didn't anyone else hit this.


> Of course, the truth is that I had been hoping to break Johannes's
> patchset in mmotm, was thrilled to get this on that, then despondent
> to realize that the only bug I had found was mine.  Surprised I've
> not seen it before in 2.5 years: tried again on 3.14-rc1, got the
> same after 25 minutes.  Probably not serious enough for -stable,
> but please can we slip the fix into 3.14 - sorry, Johannes's
> mm-keep-page-cache-radix-tree-nodes-in-check.patch will need a refresh.

I fixed it up.

unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item)
{
	struct radix_tree_node *node;
	unsigned long max_index;
	unsigned long cur_index = 0;
	unsigned long found_index = -1;

	do {
		rcu_read_lock();
		node = rcu_dereference_raw(root->rnode);
		if (!radix_tree_is_indirect_ptr(node)) {
			rcu_read_unlock();
			if (node == item)
				found_index = 0;
			break;
		}

		node = indirect_to_ptr(node);
		max_index = radix_tree_maxindex(node->path &
						RADIX_TREE_HEIGHT_MASK);
		if (cur_index > max_index) {
			rcu_read_unlock();
			break;
		}

		cur_index = __locate(node, item, cur_index, &found_index);
		rcu_read_unlock();
		cond_resched();
	} while (cur_index != 0 && cur_index <= max_index);

	return found_index;
}

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