On Tue, Dec 08, 2020 at 06:52:30PM +0100, Frederic Weisbecker wrote: > 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. I thought that you would call out unnecessarily delaying other softirq handlers. ;-) But if such delays are a problem (and they might well be), then to avoid them on non-rcu_nocb CPUs would instead/also require changing the early-exit checks to check for other pending softirqs to the existing checks involving time, need_resched, and idle. At which point, entering and exiting BH-disabled again doesn't help, other than your point about the difference in BH-disabled scopes on rcu_nocb and non-rcu_nocb CPUs. Would it make sense to exit rcu_do_batch() if more than some amount of time had elapsed and there was some non-RCU softirq pending? My guess is that the current tlimit checks in rcu_do_batch() make this unnecessary. Thoughts? Thanx, Paul
