linux-kernel - Re: [PATCHSET driver-core-next] kernfs: Protect kernfs_find_and_get_node_by

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Date: Wed, 10 Jan 2024 16:52:02 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Tejun Heo <tj@...nel.org>
Cc: andrea.righi@...onical.com, ast@...nel.org,
	linux-kernel@...r.kernel.org, geert@...ux-m68k.org
Subject: Re: [PATCHSET driver-core-next] kernfs: Protect
 kernfs_find_and_get_node_by_id() with RCU

On Tue, Jan 09, 2024 at 11:48:01AM -1000, Tejun Heo wrote:
> The BPF helper bpf_cgroup_from_id() calls kernfs_find_and_get_node_by_id()
> which acquires kernfs_idr_lock, which is an non-raw non-IRQ-safe lock.
> kernfs_idr_lock used to be a non-irq-safe lock which could lead to deadlocks
> as bpf_cgroup_from_id() can be called from any BPF programs including e.g.
> the ones that attach to functions which are holding the scheduler rq lock.
> 
> To resolve the situation dad3fb67ca1c ("kernfs: convert kernfs_idr_lock to
> an irq safe raw spinlock") converted kernfs_idr_lock to an irq-safe raw
> spinlock. However, this was also broken as we call idr_alloc*() while
> holding the lock and idr itself uses an non-irq-safe lock and also calls
> into memory allocator.
> 
> Let's instead RCU protect kernfs_node and kernfs_root so that
> kernfs_find_and_get_node_by_id() can use rcu_read_lock() instead of
> kernfs_idr_lock. While this unfortunately increases the size of kernfs_node,
> it's the most straightforward thing to do and there likely are other places
> that can take advantage of RCU protection and improve scalability too.
> 
> Please see the patch descriptions for more details.
> 
> This patchset is on top of the current driver-core-next - dad3fb67ca1c
> ("kernfs: convert kernfs_idr_lock to an irq safe raw spinlock"), and also
> available in the following git branch.
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git kernfs-use-rcu

Thanks, I'll take the first patch now, for -rc1, and then the other 2
after that to give some testing time.

greg k-h