srcu: Use invalid initial value for srcu_node GP sequence numbers

Currently, tree SRCU relies on the srcu_node structures being initialized at the same time that the srcu_struct itself is initialized, and thus use the initial grace-period sequence number as the initial value for the srcu_node structure's ->srcu_have_cbs[] and ->srcu_gp_seq_needed_exp fields. Although this has a high probability of also working when the srcu_node array is allocated and initialized at some random later time, it would be better to avoid leaving such things to chance. This commit therefore initializes these fields with 0x2, which is a recognizable invalid value. It then adds the required checks for this invalid value in order to avoid confusion on long-running kernels (especially those on 32-bit systems) that allocate and initialize srcu_node arrays late in life.

Paul E. McKenney committed 4 years ago

Commit cbdc98e1 parent aeb9b391 file · +27 −9Browse files

Unified · Split

kernel/rcu/srcutree.c+27−9

 	}
 }
+/* Invalid seq state, used during snp node initialization */
+#define SRCU_SNP_INIT_SEQ		0x2
+/*
+ * Check whether sequence number corresponding to snp node,
+ * is invalid.
+ */
+static inline bool srcu_invl_snp_seq(unsigned long s)
+{
+	return rcu_seq_state(s) == SRCU_SNP_INIT_SEQ;
+}
 /*
  * Allocated and initialize SRCU combining tree.  Returns @true if
  * allocation succeeded and @false otherwise.
 		WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
 			     ARRAY_SIZE(snp->srcu_data_have_cbs));
 		for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
-			snp->srcu_have_cbs[i] = 0;
+			snp->srcu_have_cbs[i] = SRCU_SNP_INIT_SEQ;
 			snp->srcu_data_have_cbs[i] = 0;
 		}
-		snp->srcu_gp_seq_needed_exp = 0;
+		snp->srcu_gp_seq_needed_exp = SRCU_SNP_INIT_SEQ;
 		snp->grplo = -1;
 		snp->grphi = -1;
 		if (snp == &ssp->node[0]) {
  */
 static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 {
-	if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
+	if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
-			 READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
 		return 0;
 	return SRCU_INTERVAL;
 }
 	int idx;
 	unsigned long mask;
 	struct srcu_data *sdp;
+	unsigned long sgsne;
 	struct srcu_node *snp;
 	int ss_state;
 				cbs = snp->srcu_have_cbs[idx] == gpseq;
 			snp->srcu_have_cbs[idx] = gpseq;
 			rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
-			if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
+			sgsne = snp->srcu_gp_seq_needed_exp;
+			if (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, gpseq))
 				WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
 			mask = snp->srcu_data_have_cbs[idx];
 			snp->srcu_data_have_cbs[idx] = 0;
 				  unsigned long s)
 {
 	unsigned long flags;
+	unsigned long sgsne;
 	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) ||
-			    ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
+			    (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
 				return;
 			spin_lock_irqsave_rcu_node(snp, flags);
-			if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
+			sgsne = snp->srcu_gp_seq_needed_exp;
+			if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)) {
 				spin_unlock_irqrestore_rcu_node(snp, flags);
 				return;
 			}
 {
 	unsigned long flags;
 	int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
+	unsigned long sgsne;
 	struct srcu_node *snp;
 	struct srcu_node *snp_leaf = smp_load_acquire(&sdp->mynode);
 	unsigned long snp_seq;
 				return; /* GP already done and CBs recorded. */
 			spin_lock_irqsave_rcu_node(snp, flags);
 			snp_seq = snp->srcu_have_cbs[idx];
-			if (ULONG_CMP_GE(snp_seq, s)) {
+			if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(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);
 			snp->srcu_have_cbs[idx] = s;
 			if (snp == snp_leaf)
 				snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
-			if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
+			sgsne = snp->srcu_gp_seq_needed_exp;
+			if (!do_norm && (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, s)))
 				WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
 			spin_unlock_irqrestore_rcu_node(snp, flags);
 		}