Re: [PATCH net v4] net: lapb: Add locking to the lapb module

[Xie He <xie.he.0141@gmail.com>]

With this patch, there is still a problem that lapb_unregister may run
concurrently with other LAPB API functions (such as
lapb_data_received). This other LAPB API function can get the
lapb->lock after lapb->lock is released by lapb_unregister, and
continue to do its work. This is not correct.

We can fix this problem by adding a new field "bool stop" to "struct
lapb_cb" (just like "bool t1timer_stop, t2timer_stop"), and make every
API function abort whenever it sees lapb->stop == true after getting
the lock.

Alternatively we can also require the callers (the LAPB drivers) to
never call lapb_unregister concurrently with other LAPB APIs. They
should make sure all LAPB API functions are only called after
lapb_register ends and before lapb_unregister starts. This is a
reasonable requirement, because if they don't follow this requirement,
even if we do the fix in the LAPB module (as said above), the LAPB
driver will still get the "LAPB_BADTOKEN" error from the LAPB module.
This is not desirable and I think LAPB drivers should avoid this from
happening.

So I think this problem may not need to be fixed here in the LAPB
module because the LAPB drivers should deal with this problem anyway.

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