Commit 1bafbfb3 authored by Pingfan Liu's avatar Pingfan Liu Committed by Paul E. McKenney
Browse files

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: default avatarPingfan 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: default avatarPaul E. McKenney <paulmck@kernel.org>
parent efa3c40c

kernel/rcu/srcutree.c

+7 −4
Original line number Diff line number Diff line
@@ -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);