On Thu, Jul 29, 2021 at 04:23:08PM +0800, Boqun Feng wrote:
> On Wed, Jul 21, 2021 at 01:21:18PM -0700, Paul E. McKenney wrote:
> > This commit marks some interrupt-induced read-side data races in
> > __srcu_read_lock(), __srcu_read_unlock(), and srcu_torture_stats_print().
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> > include/linux/srcutiny.h | 8 ++++----
> > kernel/rcu/srcutiny.c | 2 +-
> > 2 files changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index 0e0cf4d6a72a0..6cfaa0a9a9b96 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -61,7 +61,7 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp)
> > int idx;
> >
> > idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;
> > - WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
> > + WRITE_ONCE(ssp->srcu_lock_nesting[idx], READ_ONCE(ssp->srcu_lock_nesting[idx]) + 1);
> > return idx;
> > }
> >
> > @@ -81,11 +81,11 @@ static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
> > {
> > int idx;
> >
> > - idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;
> > + idx = ((data_race(READ_ONCE(ssp->srcu_idx)) + 1) & 0x2) >> 1;
>
> This looks very weird, any explanation why we want to put data_race() on
> a READ_ONCE()?
We don't want KCSAN to check this read, but we also don't want the
compiler to mess it up.
Thanx, Paul
> Regards,
> Boqun
>
> > pr_alert("%s%s Tiny SRCU per-CPU(idx=%d): (%hd,%hd)\n",
> > tt, tf, idx,
> > - READ_ONCE(ssp->srcu_lock_nesting[!idx]),
> > - READ_ONCE(ssp->srcu_lock_nesting[idx]));
> > + data_race(READ_ONCE(ssp->srcu_lock_nesting[!idx])),
> > + data_race(READ_ONCE(ssp->srcu_lock_nesting[idx])));
> > }
> >
> > #endif
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 26344dc6483b0..a0ba2ed49bc61 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -96,7 +96,7 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
> > */
> > void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
> > {
> > - int newval = ssp->srcu_lock_nesting[idx] - 1;
> > + int newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
> >
> > WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
> > if (!newval && READ_ONCE(ssp->srcu_gp_waiting))
> > --
> > 2.31.1.189.g2e36527f23
> >
