linux-kernel - Re: [PATCH AUTOSEL 5.4 7/7] lib/generic-radix-tree.c: Fix rare race in __genradix_ptr

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Message-ID: <g7gqwenbskp5wi7yljoaqdadmkjddouu4sez5fzryo35pu353i@fhyang5gfv7e>
Date: Sat, 12 Oct 2024 14:54:01 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Sasha Levin <sashal@...nel.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.4 7/7] lib/generic-radix-tree.c: Fix rare race
 in __genradix_ptr_alloc()

On Sat, Oct 12, 2024 at 07:29:42AM GMT, Sasha Levin wrote:
> From: Kent Overstreet <kent.overstreet@...ux.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@...ux.dev>
> Signed-off-by: Sasha Levin <sashal@...nel.org>

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