On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote: > > This is not going to work together with the "wait context validator" > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about > > printk() which is why it is still disabled by default. > > Fixing that should be "interesting". In particular, RCU CPU stall > warnings rely on the raw spin lock to reduce false positives due > to race conditions. Some thought will be required here. I don't get this part. Can you explain/give me an example where to look at? > > So assume that this is fixed and enabled then on !PREEMPT_RT it will > > complain that you have a raw_spinlock_t acquired (the one from patch > > 02/17) and attempt to acquire a spinlock_t in the memory allocator. > > Given that the slab allocator doesn't acquire any locks until it gets > a fair way in, wouldn't it make sense to allow a "shallow" allocation > while a raw spinlock is held? This would require yet another GFP_ flag, > but that won't make all that much of a difference in the total. ;-) That would be one way of dealing with. But we could go back to spinlock_t and keep the memory allocation even for RT as is. I don't see a downside of this. And we would worry about kfree_rcu() from real IRQ-off region once we get to it. > Thanx, Paul > > > > bnode = (struct kfree_rcu_bulk_data *) > > > __get_free_page(GFP_NOWAIT | __GFP_NOWARN); > > > } Sebastian
