Re: [PATCH net-next] ppp: Replace per-CPU recursion counter with lock-owner field

[Paolo Abeni <pabeni@redhat.com>]

On 6/27/25 12:50 PM, Sebastian Andrzej Siewior wrote:
> The per-CPU variable ppp::xmit_recursion is protecting against recursion
> due to wrong configuration of the ppp channels. The per-CPU variable
> relies on disabled BH for its locking. Without per-CPU locking in
> local_bh_disable() on PREEMPT_RT this data structure requires explicit
> locking.
> 
> The ppp::xmit_recursion is used as a per-CPU boolean. The counter is
> checked early in the send routing and the transmit path is only entered
> if the counter is zero. Then the counter is incremented to avoid
> recursion. It used to detect recursion on channel::downl and
> ppp::wlock.
> 
> Replace the per-CPU ppp:xmit_recursion counter with an explicit owner
> field for both structs.
> pch_downl_lock() is helper to check for recursion on channel::downl and
> either assign the owner field if there is no recursion.
> __ppp_channel_push() is moved into ppp_channel_push() and gets the
> recursion check unconditionally because it is based on the lock now.
> The recursion check in ppp_xmit_process() is based on ppp::wlock which
> is acquired by ppp_xmit_lock(). The locking is moved from
> __ppp_xmit_process() into ppp_xmit_lock() to check the owner, lock and
> then assign the owner in one spot.
> The local_bh_disable() in ppp_xmit_lock() can be removed because
> ppp_xmit_lock() disables BH as part of the locking.
> 
> Cc: Gao Feng <gfree.wind@vip.163.com>
> Cc: Guillaume Nault <g.nault@alphalink.fr>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Is there any special reason to not use local_lock here? I find this
patch quite hard to read and follow, as opposed to the local_lock usage
pattern. Also the fact that the code change does not affect RT enabled
build only is IMHO a negative thing.

/P