On Sun, Sep 13, 2026 at 2:21 PM Zqiang <[email protected]> wrote:
>
> >
> > On Sat, Sep 12, 2026 at 08:37:33PM +0800, KunWu Chan wrote:
> >
> > >
> > > On Sat, Sep 12, 2026 at 12:47 AM Paul E. McKenney <[email protected]>
> > > wrote:
> > >
> > > On Fri, Sep 11, 2026 at 05:35:15PM +0800, Kunwu Chan wrote:
> > > > Add the lockdep annotation, same-type SRCU nesting warning, and
> > > > early-boot check used by __synchronize_srcu().
> > > >
> > > > Suggested-by: Zqiang <[email protected]>
> > > > Signed-off-by: Kunwu Chan <[email protected]>
> > >
> > > Queued for review and testing, thank you both!
> > >
> > > Interestingly enough, it is now the case that there is a grace-period
> > > wait that can be placed in a normal RCU read-side critical section.
> > > Does this mean that we should also adjust the --do-srcu-lockdep testing
> > > in tools/testing/selftests/rcutorture/bin/torture.sh?
> > >
> > > Thanks, Paul. Good point.
> > >
> > > I’ll check the current `--do-srcu-lockdep` coverage, including the
> > > case where `synchronize_srcu_atomic()` is called from a normal RCU
> > > read-side critical section, and follow up with the necessary torture
> > > testing changes.
> > >
> > Sounds good!
> >
> > Perhaps you and Zqiang can work together on this. Co-developed-by,
> > for example.
>
> Hi, Paul and KunWu
Hi Zqiang,
Thanks for pointing out these cases.
>
> Should we also consider the following situations ?
>
>
> idx = srcu_read_lock_atomic(srcu)
>
> by interrupt run hardirq context:
> synchronize_rcu_atomic(srcu)
>
> srcu_read_unlock_atomic(srcu, idx)
For the same-CPU interrupt case, the existing check in
synchronize_srcu_atomic() already catches it:
synchronize_srcu_atomic(ssp)
RCU_LOCKDEP_WARN(lockdep_is_held(ssp),
"Illegal synchronize_srcu_atomic() in same-type SRCU ...");
lockdep_is_held() resolves to __lock_is_held() (lockdep.c:5612),
which checks current->held_locks[]. The interrupt handler runs with
the same current, so it sees the SRCU dep_map acquired by
srcu_read_lock_atomic().
>
>
> or:
>
>
>
> CPU0:
> CPU1:
>
> idx = srcu_read_lock_atomic(srcu)
>
> smp_call_function_single(CPU1, som_func, NULL, 1)
> to send IPI to CPU1, and sync wait complete.
> hardirq
> context or ide task context:
>
>
> some_func()
>
> ->synchronize_rcu_atomic(srcu)
>
> srcu_read_unlock_atomic(srcu, idx)
The cross-CPU case is different. current->held_locks[] is part of
struct task_struct (sched.h:1302), so the existing
__lock_is_held() check can only see the current task's held locks.
It cannot see the SRCU read-side lock held by the task running on
another CPU. The same limitation applies to the
lock_is_held(&rcu_lock_map) check in synchronize_srcu() at
srcutree.c:1665.
>
>
> Add WARN_ON(irqs_disabled()) to synchronize_rcu_atomic() ?
>
> Any thoughts?
WARN_ON(irqs_disabled()) wouldn't help with the cross-CPU case:
CPU1 could be running in process context with interrupts enabled, so
the WARN would not trigger. It would also add a false positive for
legitimate hardirq calls. synchronize_srcu_atomic() omits
might_sleep() (compare __synchronize_srcu() at srcutree.c:1676)
because it is designed to work in contexts where sleeping is not
allowed, including hardirq context.
Whether a general cross-CPU read-side-hold check is feasible is an
open question. It would need to account for the read-side state across
CPUs without adding too much overhead to the SRCU read-side fast path.
I'm happy to discuss and explore whether there is a reasonable way to
handle this cross-CPU case.
Thanks,
Kunwu
>
> Thanks
> Zqiang
>
>
> >
> > Thanx, Paul
> >
> > >
> > > Thanks,
> > > Kunwu
> > >
> > >
> > > Thanx, Paul
> > >
> > > > ---
> > > > kernel/rcu/srcutree.c | 8 ++++++++
> > > > 1 file changed, 8 insertions(+)
> > > >
> > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > index 6a9c432a3bd0..6c729e805fb3 100644
> > > > --- a/kernel/rcu/srcutree.c
> > > > +++ b/kernel/rcu/srcutree.c
> > > > @@ -2123,6 +2123,14 @@ void synchronize_srcu_atomic(struct srcu_struct
> > > *ssp)
> > > > unsigned long rdm0, rdm1;
> > > > unsigned long unlocks0, unlocks1;
> > > >
> > > > + srcu_lock_sync(&ssp->dep_map);
> > > > +
> > > > + RCU_LOCKDEP_WARN(lockdep_is_held(ssp),
> > > > + "Illegal synchronize_srcu_atomic() in same-type SRCU read-side
> > > critical section");
> > > > +
> > > > + if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
> > > > + return;
> > > > +
> > > > // Initialize. Either init_srcu_struct() was invoked or
> > > > // DEFINE_SRCU() or similar was used. Therefore, no allocation
> > > > // will be done here.
> > > > --
> > > > 2.43.0
> > > >
> > >
> >
