Le Sun, Oct 01, 2023 at 07:57:14AM +0530, Neeraj upadhyay a écrit :
>
> But "more" only checks for CBs in DONE tail. The callbacks which have been just
> accelerated are not advanced to DONE tail.
>
> Having said above, I am still trying to figure out, which callbacks
> are actually being pointed
> by NEXT tail. Given that __call_srcu() already does a advance and
> accelerate, all enqueued
> callbacks would be in either WAIT tail (the callbacks for current
> active GP) or NEXT_READY
> tail (the callbacks for next GP after current one completes). So, they
> should already have
> GP num assigned and srcu_invoke_callbacks() won't accelerate them.
> Only case I can
> think of is, if current GP completes after we sample
> rcu_seq_current(&ssp->srcu_gp_seq) for
> rcu_segcblist_advance() (so, WAIT tail cbs are not moved to DONE tail)
> and a new GP is started
> before we take snapshot ('s') of next GP for
> rcu_segcblist_accelerate(), then the gp num 's'
> > gp num of NEXT_READY_TAIL and will be put in NEXT tail. Not sure
> if my understanding is correct here. Thoughts?
>
> __call_srcu()
> rcu_segcblist_advance(&sdp->srcu_cblist,
> rcu_seq_current(&ssp->srcu_gp_seq));
> s = rcu_seq_snap(&ssp->srcu_gp_seq);
> (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
Good point! This looks plausible.
Does the (buggy) acceleration in srcu_invoke_callbacks() exists solely
to handle that case? Because then the acceleration could be moved before
the advance on callbacks handling, something like:
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 20d7a238d675..af9d8af1d321 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1245,6 +1245,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
s = rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq);
+ /*
+ * Acceleration might fail if the preceding call to
+ * rcu_segcblist_advance() also failed due to a prior incomplete grace
+ * period. This should be later fixed in srcu_invoke_callbacks().
+ */
(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
sdp->srcu_gp_seq_needed = s;
@@ -1692,6 +1697,13 @@ static void srcu_invoke_callbacks(struct work_struct *work)
ssp = sdp->ssp;
rcu_cblist_init(&ready_cbs);
spin_lock_irq_rcu_node(sdp);
+ /*
+ * Acceleration might have failed in srcu_gp_start_if_needed() if
+ * the preceding call to rcu_segcblist_advance() also failed due to
+ * a prior incomplete grace period.
+ */
+ (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
+ sdp->srcu_gp_seq_needed);
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
if (sdp->srcu_cblist_invoking ||
@@ -1720,8 +1732,6 @@ static void srcu_invoke_callbacks(struct work_struct *work)
*/
spin_lock_irq_rcu_node(sdp);
rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
- (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
- rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq));
sdp->srcu_cblist_invoking = false;
more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
spin_unlock_irq_rcu_node(sdp);
>
> Thanks
> Neeraj
>
>
> > if (more) {
> > srcu_schedule_cbs_sdp(sdp, 0);
> > }
> >
> > thanks,
> >
> > - Joel
