On Thu, Jul 12, 2018 at 01:04:27PM -0400, Alan Stern wrote: > On Thu, 12 Jul 2018, Peter Zijlstra wrote: > > > > But again, these are stuble patterns, and my guess is that several/ > > > most kernel developers really won't care about such guarantees (and > > > if some will do, they'll have the tools to figure out what they can > > > actually rely on ...) > > > > Yes it is subtle, yes most people won't care, however the problem is > > that it is subtly the wrong way around. People expect causality, this is > > a human failing perhaps, but that's how it is. > > > > And I strongly feel we should have our locks be such that they don't > > subtly break things. > > > > Take for instance the pattern where RCU relies on RCsc locks, this is an > > entirely simple and straight forward use of locks, yet completely fails > > on this subtle point. > > Do you happen to remember exactly where in the kernel source this > occurs?
Look for the uses of raw_spin_lock_irq_rcu_node() and friends in kernel/rcu and include/linux/*rcu*, along with the explanation in Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html I must confess that I am not following exactly what Peter is calling out as the failure. My best guess is that he is leading up to his call for RCsc locks, but I might have missed a turn. But Peter did supply raw_spin_lock_irq_rcu_node(), which is a marked improvement over the earlier open-coding of smp_mb__after_unlock_lock() after each and every acquisition of the rcu_node structure's ->lock, so overall I cannot complain. ;-) Thanx, Paul