On 2025-07-17 09:14, Mathieu Desnoyers wrote:
On 2025-07-16 18:54, Paul E. McKenney wrote:
[...]
2) I think I'm late to the party in reviewing srcu-fast, I'll go have a look :)
OK, I'll bite. :) Please let me know where I'm missing something: Looking at srcu-lite and srcu-fast, I understand that they fundamentally depend on a trick we published here https://lwn.net/Articles/573497/ "The RCU-barrier menagerie" that allows turning, e.g. this Dekker: volatile int x = 0, y = 0 CPU 0 CPU 1 x = 1 y = 1 smp_mb smp_mb r2 = y r4 = x BUG_ON(r2 == 0 && r4 == 0) into volatile int x = 0, y = 0 CPU 0 CPU 1 rcu_read_lock() x = 1 y = 1 synchronize_rcu() r2 = y r4 = x rcu_read_unlock() BUG_ON(r2 == 0 && r4 == 0) So looking at srcu-fast, we have: * Note that both this_cpu_inc() and atomic_long_inc() are RCU read-side * critical sections either because they disables interrupts, because they * are a single instruction, or because they are a read-modify-write atomic * operation, depending on the whims of the architecture. It appears to be pairing, as RCU read-side: - irq off/on implied by this_cpu_inc - atomic - single instruction with synchronize_rcu within the grace period, and hope that this behaves as a smp_mb pairing preventing the srcu read-side critical section from leaking out of the srcu read lock/unlock. I note that there is a validation that rcu_is_watching() within __srcu_read_lock_fast, but it's one thing to have rcu watching, but another to have an actual read-side critical section. Note that preemption, irqs, softirqs can very well be enabled when calling __srcu_read_lock_fast. My understanding of the how memory barriers implemented with RCU work is that we need to surround the memory accesses on the fast-path (where we turn smp_mb into barrier) with an RCU read-side critical section to make sure it does not spawn across a synchronize_rcu. What I am missing here is how can a RCU side-side that only consist of the irq off/on or atomic or single instruction cover all memory accesses we are trying to order, namely those within the srcu critical section after the compiler barrier() ? Is having RCU watching sufficient to guarantee this ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
