On Wed, Nov 23, 2022 at 11:20:46AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 23, 2022 at 09:56:38PM +0800, Pingfan Liu wrote:
> > Since the srcu read lock is still held during srcu_funnel_gp_start(),
> > the seq snap should be the largest number for the slot
> > srcu_have_cbs[idx].
> >
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > To: rcu@xxxxxxxxxxxxxxx
> > ---
> > include/linux/rcupdate.h | 1 +
> > kernel/rcu/srcutree.c | 11 ++++++-----
> > 2 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 08605ce7379d..a09007236660 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -32,6 +32,7 @@
> > #include <linux/context_tracking_irq.h>
> >
> > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > +#define ULONG_CMP_GT(a, b) (ULONG_MAX / 2 > (a) - (b))
>
> Please see below...
>
> > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > #define ulong2long(a) (*(long *)(&(a)))
> > #define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 2fc0e775ade4..41902b823687 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -905,14 +905,15 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
> > spin_lock_irqsave_rcu_node(snp, flags);
> > snp_seq = snp->srcu_have_cbs[idx];
> > - if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
> > + /*
> > + * s should be the biggest in the current slot. Hence only LE is
> > + * valid
> > + */
> > + BUG_ON(ULONG_CMP_GT(snp_seq, s));
>
> Why not this? (Plus adjusting the comment above, of course.)
>
> WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq));
>
A neat solution!
And what about the comment
/* s should be the biggest in the current slot. */
> That way we don't need ULONG_CMP_GT(). Plus if we are confused about
> s being the biggest in the current slot, we get a splat and can debug
> further. We both might be quite sure that we are not confused, but
> that is exactly when we are most prone to making mistakes. ;-)
>
Same feeling :)
> I also ask that you run with this check for some time. After all, if
Sure. I will try it immediately.
> the assumption is incorrect, the resulting SRCU hangs off in various
> systems around the world will not be so much fun to debug.
>
> Given that this is slowpath code, it is much better to take an extra
> compare and branch than to introduce even an extremely small risk of
> hanging SRCU.
>
Agree.
Thanks,
Pingfan
> Thanx, Paul
>
> > + if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) {
> > if (snp == snp_leaf && snp_seq == s)
> > snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
> > spin_unlock_irqrestore_rcu_node(snp, flags);
> > - if (snp == snp_leaf && snp_seq != s) {
> > - srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
> > - return;
> > - }
> > if (!do_norm)
> > srcu_funnel_exp_start(ssp, snp, s);
> > return;
> > --
> > 2.31.1
> >
