On Tue, Aug 11, 2020 at 05:43:16PM +0200, Thomas Gleixner wrote: > "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes: > > On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote: > >> Now RCU creates a new thing which enforces to make page allocation in > >> atomic context possible on RT. What for? > >> > >> What's the actual use case in truly atomic context for this new thing on > >> an RT kernel? > > > > It is not just RT kernels. CONFIG_PROVE_RAW_LOCK_NESTING=y propagates > > this constraint to all configurations, and a patch in your new favorite > > subsystem really did trigger this lockdep check in a non-RT kernel. > > > >> The actual RCU code disabling interrupts is an implementation detail > >> which can easily be mitigated with a local lock. > > > > In this case, we are in raw-spinlock context on entry to kfree_rcu(). > > Where? Some BPF code that needs to process and free a list. As noted above, this is a patch rather than something that is already in mainline. Not surprising, though, given call_rcu() invocations in similar contexts. Yes, we can perhaps rework all current and future callers to avoid invoking both call_rcu() and kfree_rcu() from raw atomic context, but the required change to permit this is quite a bit simpler. Thanx, Paul
