>
> On Thu, Sep 10, 2026 at 8:16 PM Zqiang <[email protected]> wrote:
>
> >
> > From: Kunwu Chan <[email protected]>
> >
> > 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:
> >
> Hi Zqiang
> Sent as:
> https://lore.kernel.org/rcu/[email protected]/
>
> Did a small late_initcall that enters an atomic read-side critical section and
> calls synchronize_srcu_atomic(). srcu_lock_sync() catches it:
> WARNING: possible recursive locking detected
> swapper/0/1 is trying to acquire lock: (&ss) at:
> synchronize_srcu_atomic+0x28/0x380
> but task is already holding lock: (&ss) at:
> srcu_lockdep_test_init+0xd0/0x170
> *** DEADLOCK ***
> Rcutorture SRCU-N/P also pass.
Hi, kunwu
You can add srcu_atomic lockdep test to rcutorture to verify
your change, and I also saw that Paul suggested the same thing :)
For example, as shown below (I am not test or verify it):
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index d795e07a12ad..d4f7399acbd5 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -4662,6 +4662,17 @@ static DECLARE_RWSEM(rwsem7);
static DECLARE_RWSEM(rwsem8);
static DECLARE_RWSEM(rwsem9);
+static DEFINE_RAW_SPINLOCK(spin0);
+static DEFINE_RAW_SPINLOCK(spin1);
+static DEFINE_RAW_SPINLOCK(spin2);
+static DEFINE_RAW_SPINLOCK(spin3);
+static DEFINE_RAW_SPINLOCK(spin4);
+static DEFINE_RAW_SPINLOCK(spin5);
+static DEFINE_RAW_SPINLOCK(spin6);
+static DEFINE_RAW_SPINLOCK(spin7);
+static DEFINE_RAW_SPINLOCK(spin8);
+static DEFINE_RAW_SPINLOCK(spin9);
+
DEFINE_STATIC_SRCU(srcu0);
DEFINE_STATIC_SRCU(srcu1);
DEFINE_STATIC_SRCU(srcu2);
@@ -4673,6 +4684,17 @@ DEFINE_STATIC_SRCU(srcu7);
DEFINE_STATIC_SRCU(srcu8);
DEFINE_STATIC_SRCU(srcu9);
+DEFINE_STATIC_SRCU_ATOMIC(srcu0_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu1_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu2_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu3_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu4_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu5_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu6_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu7_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu8_atomic);
+DEFINE_STATIC_SRCU_ATOMIC(srcu9_atomic);
+
static int srcu_lockdep_next(const char *f, const char *fl, const char *fs,
const char *fu, int i,
int cyclelen, int deadlock)
{
@@ -4702,6 +4724,13 @@ static void rcu_torture_init_srcu_lockdep(void)
&rwsem5, &rwsem6, &rwsem7, &rwsem8,
&rwsem9 };
struct srcu_struct *srcus[] = { &srcu0, &srcu1, &srcu2, &srcu3, &srcu4,
&srcu5, &srcu6, &srcu7, &srcu8, &srcu9
};
+
+ raw_spinlock_t *spins[] = { &spin0, &spin1, &spin2, &spin3, &spin4,
&spin5,
+ &spin6, &spin7, &spin8, &spin9};
+ struct srcu_struct *srcus_atomic[] = {&srcu0_atomic, &srcu1_atomic,
&srcu2_atomic,
+ &srcu3_atomic, &srcu4_atomic,
&srcu5_atomic,
+ &srcu6_atomic, &srcu7_atomic,
&srcu8_atomic,
+ &srcu9_atomic};
int testtype;
if (!test_srcu_lockdep)
@@ -4812,11 +4841,48 @@ static void rcu_torture_init_srcu_lockdep(void)
}
#endif // #ifdef CONFIG_TASKS_TRACE_RCU
+ if (testtype == 4) {
+ pr_info("%s: test_srcu_lockdep = %05d: SRCU_ATOMIC %d-way
%sdeadlock.\n",
+ __func__, test_srcu_lockdep, cyclelen, deadlock ? "" :
"non-");
+ if (deadlock && cyclelen == 1)
+ pr_info("%s: Expect hang.\n", __func__);
+ for (i = 0; i < cyclelen; i++) {
+ j = srcu_lockdep_next(__func__,
"srcu_read_lock_atomic", "synchronize_srcu_atomic",
+ "srcu_read_unlock_atomic", i,
cyclelen, deadlock);
+ idx = srcu_read_lock_atomic(srcus_atomic[i]);
+ if (j >= 0)
+ synchronize_srcu_atomic(srcus_atomic[j]);
+ srcu_read_unlock_atomic(srcus_atomic[i], idx);
+ }
+ return;
+ }
+
+ if (testtype == 1) {
+ pr_info("%s: test_srcu_lockdep = %05d:
SRCU_ATOMIC/raw_spinlock %d-way %sdeadlock.\n",
+ __func__, test_srcu_lockdep, cyclelen, deadlock ? "" :
"non-");
+ for (i = 0; i < cyclelen; i++) {
+ pr_info("%s: srcu_read_lock_atomic(%d),
raw_spin_lock(%d), raw_spin_unlock(%d), srcu_read_unlock_atomic(%d)\n",
+ __func__, i, i, i, i);
+ idx = srcu_read_lock_atomic(srcus_atomic[i]);
+ raw_spin_lock(spins[i]);
+ raw_spin_unlock(spins[i]);
+ srcu_read_unlock_atomic(srcus_atomic[i], idx);
+
+ j = srcu_lockdep_next(__func__, "raw_spin_lock",
"synchronize_srcu_atomic",
+ "raw_spin_unlock", i, cyclelen,
deadlock);
+ raw_spin_lock(spins[i]);
+ if (j >= 0)
+ synchronize_srcu_atomic(srcus_atomic[j]);
+ raw_spin_unlock(spins[i]);
+ }
+ return;
+ }
+
err_out:
pr_info("%s: test_srcu_lockdep = %05d does nothing.\n", __func__,
test_srcu_lockdep);
pr_info("%s: test_srcu_lockdep = DNNL.\n", __func__);
pr_info("%s: D: Deadlock if nonzero.\n", __func__);
- pr_info("%s: NN: Test number, 0=SRCU, 1=SRCU/mutex, 2=SRCU/rwsem,
3=SRCU/Tasks Trace RCU.\n", __func__);
+ pr_info("%s: NN: Test number, 0=SRCU, 1=SRCU/mutex, 2=SRCU/rwsem,
3=SRCU/Tasks Trace RCU, 4=SRCU_ATOMIC, 5=SRCU_ATOMIC/raw_spinlock.\n",
__func__);
pr_info("%s: L: Cycle length.\n", __func__);
if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU))
pr_info("%s: NN=3 disallowed because kernel is built with
CONFIG_TASKS_TRACE_RCU=n\n", __func__);
Thanks
Zqiang
>
> Thanks, Kunwu
>
> >
> > 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
> >
>
