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

["Zqiang" <qiang.zhang@linux.dev>]

> 
> From: Kunwu Chan <kunwu.chan@gmail.com>
> 
> 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



> 
> 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
>