On 2025-08-18 14:06:39 [-0400], Waiman Long wrote: > > index 80c914f6eae7a..37b6a5670c2fa 100644 > > --- a/Documentation/locking/locktypes.rst > > +++ b/Documentation/locking/locktypes.rst > > @@ -204,6 +204,27 @@ per-CPU data structures on a non PREEMPT_RT kernel. > > local_lock is not suitable to protect against preemption or interrupts on a > > PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics. > > +CPU local scope and bottom-half > > +------------------------------- > > + > > +Per-CPU variables that are accessed only in softirq context should not rely on > > +the assumption that this context is implicitly protected due to being > > +non-preemptible. In a PREEMPT_RT kernel, softirq context is preemptible, and > > +synchronizing every bottom-half-disabled section via implicit context results > > +in an implicit per-CPU "big kernel lock." > > + > > +A local_lock_t together with local_lock_nested_bh() and > > +local_unlock_nested_bh() for locking operations help to identify the locking > > +scope. > > + > > +When lockdep is enabled, these functions verify that data structure access > > +occurs within softirq context. > > +Unlike local_lock(), local_unlock_nested_bh() does not disable preemption and > > +does not add overhead when used without lockdep. > > Should it be local_lock_nested_bh()? It doesn't make sense to compare > local_unlock_nested_bh() against local_lock(). In a PREEMPT_RT kernel, > local_lock() disables migration but not preemption. Yes, it should have been the lock and not the unlock part. I mention just preemption part here because it focuses on the !RT part compared to local_lock() and that it adds no overhead. The PREEMPT_RT part below mentions that it behaves as a real lock so that should be enough (not to mention the migration part (technically migration must be already disabled so we could omit disabling migration here but it is just a counter increment/ decrement at this point so we don't win much by doing so)). I made the following: @@ -219,11 +219,11 @@ scope. When lockdep is enabled, these functions verify that data structure access occurs within softirq context. -Unlike local_lock(), local_unlock_nested_bh() does not disable preemption and +Unlike local_lock(), local_lock_nested_bh() does not disable preemption and does not add overhead when used without lockdep. On a PREEMPT_RT kernel, local_lock_t behaves as a real lock and -local_unlock_nested_bh() serializes access to the data structure, which allows +local_lock_nested_bh() serializes access to the data structure, which allows removal of serialization via local_bh_disable(). raw_spinlock_t and spinlock_t Good? > Cheers, > Longman > > > + > > +On a PREEMPT_RT kernel, local_lock_t behaves as a real lock and > > +local_unlock_nested_bh() serializes access to the data structure, which allows > > +removal of serialization via local_bh_disable(). > > raw_spinlock_t and spinlock_t > > ============================= Sebastian
