On Tue, Dec 08, 2020 at 09:19:27AM -0800, Paul E. McKenney wrote: > On Tue, Dec 08, 2020 at 04:54:57PM +0100, Frederic Weisbecker wrote: > > On Tue, Dec 08, 2020 at 06:58:10AM -0800, Paul E. McKenney wrote: > > > Hello, Frederic, > > > > > > Boqun just asked if RCU callbacks ran in BH-disabled context to avoid > > > concurrent execution of the same callback. Of course, this raises the > > > question of whether a self-posting callback can have two instances of > > > itself running concurrently while a CPU is in the process of transitioning > > > between softirq and rcuo invocation of callbacks. > > > > > > I believe that the answer is "no" because BH-disabled context is > > > an implicit RCU read-side critical section. Therefore, the initial > > > invocation of the RCU callback must complete in order for a new grace > > > period to complete, and a new grace period must complete before the > > > second invocation of that same callback to start. > > > > > > Does that make sense, or am I missing something? > > > > Sounds like a good explanation. But then why are we actually calling > > the entire rcu_do_batch() under BH-disabled context? Was it to quieten > > lockdep against rcu_callback_map ? > > Inertia and lack of complaints about it. ;-) > > Plus when called from softirq, neither local_bh_disable() nor > rcu_read_lock() is necessary, and so represents pointless overhead. > > > Wouldn't rcu_read_lock() around callbacks invocation be enough? Or is > > there another reason for the BH-disabled context that I'm missing? > > There are likely to be callback functions that use spin_lock() instead > of spin_lock_bh() because they know that they are invoked in BH-disabled > context. Ah right. So perhaps we can keep local_bh_disable() instead. > > But what does this change help? It reduces the code scope running with BH disabled. Also narrowing down helps to understand what it actually protects. Thanks.