srcu: Remove needless rcu_seq_done() check while holding read lock

The srcu_gp_start_if_needed() function now read-holds the srcu_struct
whose grace period is being started, which means that the corresponding
SRCU grace period cannot end.  This in turn means that the SRCU
grace-period sequence number returned by rcu_seq_snap() cannot expire
during this time.  And that means that the calls to rcu_seq_done() in
srcu_funnel_exp_start() and srcu_funnel_gp_start() can never return true.

This commit therefore removes these rcu_seq_done() checks, but adds checks
in kernels built with CONFIG_PROVE_RCU=y that splats if rcu_seq_done()
does somehow return true.

[ paulmck: Rearrange checks to handle kernels built with lockdep. ]

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rcu@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 6af0312005801c46cbcf9b12bdcf8288c0963a19..68b8d8b150db135522ff1b822c9ec405736ec410 100644 (file)
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -915,7 +915,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
        if (snp)
                for (; snp != NULL; snp = snp->srcu_parent) {
                        sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp);
-                       if (rcu_seq_done(&ssp->srcu_gp_seq, s) ||
+                       if (WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_gp_seq, s)) ||
                            (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
                                return;
                        spin_lock_irqsave_rcu_node(snp, flags);
@@ -942,6 +942,9 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
  *
  * Note that this function also does the work of srcu_funnel_exp_start(),
  * in some cases by directly invoking it.
+ *
+ * The srcu read lock should be hold around this function. And s is a seq snap
+ * after holding that lock.
  */
 static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
                                 unsigned long s, bool do_norm)
@@ -962,7 +965,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
        if (snp_leaf)
                /* Each pass through the loop does one level of the srcu_node tree. */
                for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
-                       if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != snp_leaf)
+                       if (WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_gp_seq, s)) && snp != snp_leaf)
                                return; /* GP already done and CBs recorded. */
                        spin_lock_irqsave_rcu_node(snp, flags);
                        snp_seq = snp->srcu_have_cbs[idx];
@@ -999,8 +1002,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
        if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
                WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 
-       /* If grace period not already done and none in progress, start it. */
-       if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
+       /* If grace period not already in progress, start it. */
+       if (!WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_gp_seq, s)) &&
            rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
                WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
                srcu_gp_start(ssp);