Re: [RFC PATCH 1/7] mm/slab: introduce k[v]free_rcu() with struct rcu_ptr

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Feb 11, 2026 at 07:44:37PM +0900, Harry Yoo wrote:
> On Wed, Feb 11, 2026 at 11:16:51AM +0100, Uladzislau Rezki wrote:
> > If this is supposed to be invoked from NMI, should we better just detect
> > such context in the kvfree_call_rcu()? There are lot of "allow_spin" checks
> > which make it easy to get lost.
> 
> Detecting if it's NMI might be okay, but IIUC re-entrancy requirement
> not only comes from NMI but also from attaching bpf programs to
> kernel functions, something like:
> 
> "Run a BPF program whenever queue_delayed_work() is called,
>  ... and the BPF program somehow frees memory via kfree_rcu_nolock()".
> 
> Then, by the time the kernel calls queue_delayed_work() while holding
> krcp->lock, it run the BPF program and calls kfree_rcu_nolock(),
> it is not allowed to spin on krcp->lock.
> 
> 
> > As i see you maintain llist and the idea is simply to re-enter to the
> > kvfree_rcu() again with allow-spin=true, since then it will be "normal"
> > context.
> 
> It tries to acquire the lock and add it to krcp->head, but if somebody
> is already holding the lock, it re-runs kvfree_rcu() with irq work.
> 
Check no_spin on entry, if true, llist_add, queue-irq-work. Re-enter.
You might need to set-up interval to prevent frequent bouncing.

--
Uladzislau Rezki