Re: [PATCH AUTOSEL 5.4 7/7] lib/generic-radix-tree.c: Fix rare race in __genradix_ptr_alloc()

Kent Overstreet
2024-10-12T18:54:11Z

On Sat, Oct 12, 2024 at 07:29:42AM GMT, Sasha Levin wrote:
> From: Kent Overstreet <kent.overstreet@linux.dev>
>
> [ Upstream commit b2f11c6f3e1fc60742673b8675c95b78447f3dae ]
>
> If we need to increase the tree depth, allocate a new node, and then
> race with another thread that increased the tree depth before us, we'll
> still have a preallocated node that might be used later.
>
> If we then use that node for a new non-root node, it'll still have a
> pointer to the old root instead of being zeroed - fix this by zeroing it
> in the cmpxchg failure path.
>
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

there wasn't any need to backport this, bcachefs is the only thing that
uses genradix in multithreaded mode