Re: [PATCH v2 04/21] srcutree: Add an atomic Tree SRCU

[David Woodhouse]

On Wed, 2026-10-07 at 13:59 -0700, Paul E. McKenney wrote:
> Some dedicated srcu_struct users have read-side critical sections
> which are short, never sleep, and never block on anything which may
> itself depend on memory allocation — because they were, until
> recently, spinlock or rwlock critical sections. For such a domain the
> update side can safely wait for readers by spinning, from contexts
> where sleeping is undesirable or the grace-period machinery's
> latency (workqueue scheduling, jiffy-paced retries) dominates the
> actual reader drain time.
> 
> But that is only safe if every reader keeps the promise. Make the
> promise explicit and machine-checkable:
> 
>  - srcu_read_lock_atomic() / srcu_read_unlock_atomic() enter the
>    usual (smp_mb-based) read-side critical section with preemption
>    disabled and (except in hardirq, where it is redundant)
>    non_block_start() armed, recording SRCU_READ_FLAVOR_ATOMIC in the
>    per-CPU reader flavor. Disabling preemption enforces the
>    no-sleeping promise on every configuration and bounds the section,
>    so it is always running on some CPU; non_block_start() extends the
>    enforcement to even potentially-sleeping calls on paths which
>    happen not to block. The existing reader-flavor consistency checks
>    complain about any mixing with other flavors.
> 
>  - synchronize_srcu_atomic() waits for all pre-existing readers by
>    repeating the try_synchronize_srcu() both-epoch counter proof with
>    cpu_relax() until it succeeds: no sleeping, no index flip, no
>    grace-period sequence update, and therefore no interaction with
>    concurrent call_srcu(), synchronize_srcu() or srcu_barrier(). It
>    always provides the full grace-period guarantee: if the domain
>    turns out to have had readers of any other flavor — a caller bug,
>    since such a reader may be asleep and spinning on it would be
>    unbounded — it complains and falls back to a real (sleeping) grace
>    period internally, that being the only correct wait for a
>    possibly-sleeping reader. The flavor mask is rechecked on every
>    iteration so a first non-atomic reader appearing mid-spin takes
>    the same path.
> 
> The immediate motivation is the proposed conversion of KVM's
> gfn_to_pfn_cache to SRCU¹, whose mmu_notifier invalidation path drains
> readers before the primary MMU zaps a page. With the readers declared
> atomic, that drain becomes spin-only: no sleeping at all in the
> notifier, bounded by the longest reader section, satisfying even the
> strictest reading of the OOM-reaper non-blocking requirement without
> needing to touch the non_block_start() annotation².
> 
> This commit also adds atomic-SRCU-specific initializers:
> init_srcu_struct_atomic(), DEFINE_SRCU_ATOMIC(), and
> DEFINE_STATIC_SRCU_ATOMIC().
> 
> This commit implements only Tree SRCU.  Tiny SRCU will follow.
> 
> ¹ https://lore.kernel.org/all/20260811132237.102400-1-dwmw2@xxxxxxxxxxxxx/
> ² https://lore.kernel.org/all/20260812134934.GC662699@xxxxxxxx/
> 
> [ paulmck: Apply Kunwu Chan feedback. ]
> 
> Co-developed-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Signed-off-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Assisted-by: Claude:claude-mythos-5
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Thank you for seeing this concept through to completion.

If it isn't already too late (i.e. if it hasn't already made it to a
tree where changing commit-ids is painful), please could you change my
attribution to use the dwmw@xxxxxxxxxxxx email address for both Co-
developed-by: and Signed-off-by: tags? I'm fairly sure you made up that
Signed-off-by: with the infradead address, which is naughty of you :)

I tell people you should only ever cut-and-paste SoB lines, because
that way the magic is intact. So here are the two you need for patches
4 and 5 of this series:

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

Thanks.

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