Re: [PATCH v2] srcu: Improve comments about acceleration leak

[Paul E. McKenney]

On Tue, Dec 12, 2023 at 03:11:30PM -0500, Joel Fernandes wrote:
> 
> 
> > On Dec 10, 2023, at 8:57 PM, Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> wrote:
> > 
> > The comments added in commit 1ef990c4b36b ("srcu: No need to
> > advance/accelerate if no callback enqueued") are a bit confusing to me.
> > The comments are describing a scenario for code that was moved and is
> > no longer the way it was (snapshot after advancing). Improve the code
> > comments to reflect this and also document by acceleration can never
> > fail.
> > 
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Neeraj Upadhyay <neeraj.iitr10@xxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> 
> Do we want to quick review and put it in Neeraj PR?
> 
> Or next merge window ok with me. Just that then I have to keep track of it ;-)

Or it could get an ack and I could pull it into -rcu.  ;-)

							Thanx, Paul

> Thanks,
> 
> - Joel 
> 
> 
> 
> > ---
> > v1->v2: Fix typo in change log.
> > 
> > kernel/rcu/srcutree.c | 24 ++++++++++++++++++++----
> > 1 file changed, 20 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 0351a4e83529..051e149490d1 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -1234,11 +1234,20 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> >    if (rhp)
> >        rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> >    /*
> > -     * The snapshot for acceleration must be taken _before_ the read of the
> > -     * current gp sequence used for advancing, otherwise advancing may fail
> > -     * and acceleration may then fail too.
> > +     * It's crucial to capture the snapshot 's' for acceleration before
> > +     * reading the current gp_seq that is used for advancing. This is
> > +     * essential because if the acceleration snapshot is taken after a
> > +     * failed advancement attempt, there's a risk that a grace period may
> > +     * conclude and a new one may start in the interim. If the snapshot is
> > +     * captured after this sequence of events, the acceleration snapshot 's'
> > +     * could be excessively advanced, leading to acceleration failure.
> > +     * In such a scenario, an 'acceleration leak' can occur, where new
> > +     * callbacks become indefinitely stuck in the RCU_NEXT_TAIL segment.
> > +     * Also note that encountering advancing failures is a normal
> > +     * occurrence when the grace period for RCU_WAIT_TAIL is in progress.
> >     *
> > -     * This could happen if:
> > +     * To see this, consider the following events which occur if
> > +     * rcu_seq_snap() were to be called after advance:
> >     *
> >     *  1) The RCU_WAIT_TAIL segment has callbacks (gp_num = X + 4) and the
> >     *     RCU_NEXT_READY_TAIL also has callbacks (gp_num = X + 8).
> > @@ -1264,6 +1273,13 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> >    if (rhp) {
> >        rcu_segcblist_advance(&sdp->srcu_cblist,
> >                      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
> > +        /*
> > +         * Acceleration can never fail because the state of gp_seq used
> > +         * for advancing is <= the state of gp_seq used for
> > +         * acceleration. This means that RCU_NEXT_TAIL segment will
> > +         * always be able to be emptied by the acceleration into the
> > +         * RCU_NEXT_READY_TAIL or RCU_WAIT_TAIL segments.
> > +         */
> >        WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s));
> >    }
> >    if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> > -- 
> > 2.43.0.472.g3155946c3a-goog
> >