Re: [PATCH 00/13] srcu: Round out atomic SRCU support

[KunWu Chan]

On Thu, Sep 10, 2026 at 8:16 PM Zqiang <qiang.zhang@xxxxxxxxx> wrote:
>
> >
> > From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> >
> > This series rounds out atomic SRCU support with a reader-free fastpath,
> > some atomic-context fixes, and additional test coverage.
> >
> > The core change is a reader-free fastpath for synchronize_srcu_atomic().
> > Atomic SRCU's read-side critical sections disable preemption, so when
> > the summed lock and unlock counts match for both ranks,
> > synchronize_srcu_atomic() can end the grace period immediately, without
> > the index flip or the srcu_advance_state() scans. Correctness depends
> > on the grace-period anchor written by srcu_gp_start() preceding the
> > lock-count scan; two LKMM litmus tests (patches 1-2) verify that
> > ordering.
>
> For synchronize_srcu_atomic(), the following items maybe also
> need to be added:
>
>         srcu_lock_sync(&ssp->dep_map);
>
>         RCU_LOCKDEP_WARN(lockdep_is_held(ssp),
>                          "Illegal synchronize_srcu() in same-type SRCU read-side critical section");
>
>         if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
>                 return;
>

Thanks, Zqiang. Good catch on these missing SRCU synchronization
checks.

I’ll review these against the existing SRCU synchronization path and
send a follow-up patch if confirmed. I’ll add your Suggested-by tag
for the changes.

Thanks,
Kunwu

> Thanks
> Zqiang
>
>
>
> >
> > The series is:
> >
> >  1-2 LKMM litmus tests for the reader-free fastpath ordering.
> >  3 srcutree: add the reader-free fastpath.
> >  4 rcutorture: add --do-atomic-srcu to torture.sh.
> >  5-7 srcutree: honor and propagate is_atomic so atomic SRCU never
> >  transitions to big.
> >  8 srcutree: forbid srcu_expedite_current() on atomic SRCU.
> >  9 rcutorture: disable srcu_expedite_current() for atomic SRCU.
> >  10 srcutree: skip callback scheduling for atomic SRCU grace periods.
> >  11 srcutree: drop the srcu_barrier() sleep for atomic SRCU.
> >  12 srcutree: remove leftover debug pr_alert()s.
> >  13 srcu: restrict the non_block annotation to task context, fixing a
> >  KCSAN data race.
> >
> > The KCSAN race in patch 13 is a data race on the interrupted task's
> > ->non_block_count when a timer callback runs atomic-SRCU readers in
> > inline softirq context.
> >
> > Tested with herd7 7.58 (litmus tests), torture.sh --do-atomic-srcu, and
> > torture.sh --do-atomic-srcu --do-kcsan (KCSAN+PROVE_LOCKING).
> >
> > Kunwu Chan (13):
> >  litmus: Add SRCU fastpath anchor-before-scan test
> >  litmus: Add SRCU fastpath scan-before-anchor test
> >  srcutree: Add reader-free fastpath to synchronize_srcu_atomic()
> >  rcutorture: Add atomic-SRCU support to torture.sh
> >  srcutree: Honor is_atomic in check_init_srcu_struct()
> >  srcutree: Make init_srcu_struct_atomic() prevent transition to big
> >  srcutree: Don't transition atomic SRCU to big in srcu_gp_end()
> >  srcutree: Forbid srcu_expedite_current() on atomic SRCU
> >  rcutorture: Disable srcu_expedite_current() for atomic SRCU
> >  srcutree: Skip callback scheduling for atomic SRCU grace periods
> >  srcutree: Remove srcu_barrier() sleep for atomic SRCU
> >  srcutree: Remove debug pr_alert()s
> >  srcu: Restrict atomic-SRCU non_block annotation to task context
> >
> >  include/linux/srcu.h | 10 +-
> >  kernel/rcu/rcutorture.c | 1 +
> >  kernel/rcu/srcutree.c | 95 ++++++++++++++-----
> >  tools/memory-model/litmus-tests/README | 16 ++++
> >  .../SRCU-fastpath-anchor-before-scan.litmus | 56 +++++++++++
> >  .../SRCU-fastpath-scan-before-anchor.litmus | 53 +++++++++++
> >  .../selftests/rcutorture/bin/torture.sh | 24 +++++
> >  7 files changed, 229 insertions(+), 26 deletions(-)
> >  create mode 100644 tools/memory-model/litmus-tests/SRCU-fastpath-anchor-before-scan.litmus
> >  create mode 100644 tools/memory-model/litmus-tests/SRCU-fastpath-scan-before-anchor.litmus
> >
> > base-commit: ef15070f96c5f07de3a9b593c36f6e83bc9e3d58
> > --
> > 2.43.0
> >