On Thu, Jul 17, 2025 at 10:46:46AM -0400, Mathieu Desnoyers wrote:
> 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 ?
Good eyes!!!
The trick is that this "RCU read-side critical section" consists only of
either this_cpu_inc() or atomic_long_inc(), with the latter only happening
in systems that have NMIs, but don't have NMI-safe per-CPU operations.
Neither this_cpu_inc() nor atomic_long_inc() can be interrupted, and
thus both act as an interrupts-disabled RCU read-side critical section.
Therefore, if the SRCU grace-period computation fails to see an
srcu_read_lock_fast() increment, its earlier code is guaranteed to
happen before the corresponding critical section. Similarly, if the SRCU
grace-period computation sees an srcu_read_unlock_fast(), its subsequent
code is guaranteed to happen after the corresponding critical section.
Does that help? If so, would you be interested and nominating a comment?
Or am I missing something subtle here?
Either way, many thanks for digging into this!!!
Thanx, Paul
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
