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The per-CPU variable ppp::xmit_recursion is protecting against recursion due to wrong configuration of the ppp unit. 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. Create a struct ppp_xmit_recursion and move the counter into it. Add local_lock_t to the struct and use local_lock_nested_bh() for locking. Due to possible nesting, the lock cannot be acquired unconditionally but it requires an owner field to identify recursion before attempting to acquire the lock. The counter is incremented and checked only after the lock is acquired. Since it functions as a boolean rather than a count, and its role is now superseded by the owner field, it can be safely removed. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Link: https://patch.msgid.link/20250715150806.700536-2-bigeasy@linutronix.de Reviewed-by: Guillaume Nault <gnault@redhat.com> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.