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Commit 29365e56 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Convert grace-period requests to ->gp_seq 



This commit converts the grace-period request code paths from ->completed
and ->gpnum to ->gp_seq.  The need_future_gp_element() macro encapsulates
the shift operation required to use ->gp_seq as an index to the
->need_future_gp[] array.  The rcu_cbs_completed() function is removed
in favor of the rcu_seq_snap() function.  The rcu_start_this_gp()
gets some temporary consistency checks and uses rcu_seq_done(),
rcu_seq_current(), rcu_seq_state(), and rcu_gp_in_progress() in place
of the earlier open-coded comparisons of ->gpnum and ->completed.
The rcu_future_gp_cleanup() function replaces use of ->completed
with ->gp_seq.  The rcu_accelerate_cbs() function replaces a call to
rcu_cbs_completed() with one to rcu_seq_snap().  The rcu_advance_cbs()
function replaces an access to >completed with one to ->gp_seq and adds
some temporary warnings.  The rcu_nocb_wait_gp() function replaces a
call to rcu_cbs_completed() with one to rcu_seq_snap() and an open-coded
comparison with rcu_seq_done().

The temporary warnings will be removed when the various ->gpnum and
->completed fields are removed.  Their purpose is to locate code who
might still be using ->gpnum and ->completed.  (Much easier that way
than trying to trace down the causes of too-short grace periods and
grace-period hangs!)

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent d43a5d32

kernel/rcu/tree.c

+20 −64
Original line number Diff line number Diff line
@@ -1548,52 +1548,6 @@ void rcu_cpu_stall_reset(void) 
		WRITE_ONCE(rsp->jiffies_stall, jiffies + ULONG_MAX / 2);

}



/*

 * Determine the value that ->completed will have at the end of the

 * next subsequent grace period.  This is used to tag callbacks so that

 * a CPU can invoke callbacks in a timely fashion even if that CPU has

 * been dyntick-idle for an extended period with callbacks under the

 * influence of RCU_FAST_NO_HZ.

 *

 * The caller must hold rnp->lock with interrupts disabled.

 */

static unsigned long rcu_cbs_completed(struct rcu_state *rsp,

				       struct rcu_node *rnp)

{

	raw_lockdep_assert_held_rcu_node(rnp);



	/*

	 * If RCU is idle, we just wait for the next grace period.

	 * But we can only be sure that RCU is idle if we are looking

	 * at the root rcu_node structure -- otherwise, a new grace

	 * period might have started, but just not yet gotten around

	 * to initializing the current non-root rcu_node structure.

	 */

	if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed)

		return rnp->completed + 1;



	/*

	 * If the current rcu_node structure believes that RCU is

	 * idle, and if the rcu_state structure does not yet reflect

	 * the start of a new grace period, then the next grace period

	 * will suffice.  The memory barrier is needed to accurately

	 * sample the rsp->gpnum, and pairs with the second lock

	 * acquisition in rcu_gp_init(), which is augmented with

	 * smp_mb__after_unlock_lock() for this purpose.

	 */

	if (rnp->gpnum == rnp->completed) {

		smp_mb(); /* See above block comment. */

		if (READ_ONCE(rsp->gpnum) == rnp->completed)

			return rnp->completed + 1;

	}



	/*

	 * Otherwise, wait for a possible partial grace period and

	 * then the subsequent full grace period.

	 */

	return rnp->completed + 2;

}



/* Trace-event wrapper function for trace_rcu_future_grace_period.  */

static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,

			      unsigned long c, const char *s)
@@ -1629,16 +1583,16 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, 
	 * not be released.

	 */

	raw_lockdep_assert_held_rcu_node(rnp);

	WARN_ON_ONCE(c & 0x2); /* Catch any lingering use of ->gpnum. */

	WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq)); /* Catch any ->completed/->gp_seq mismatches. */

	trace_rcu_this_gp(rnp, rdp, c, TPS("Startleaf"));

	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {

		if (rnp_root != rnp)

			raw_spin_lock_rcu_node(rnp_root);

		WARN_ON_ONCE(ULONG_CMP_LT(rnp_root->gpnum +

					  need_future_gp_mask(), c));

		if (need_future_gp_element(rnp_root, c) ||

		    ULONG_CMP_GE(rnp_root->gpnum, c) ||

		    rcu_seq_done(&rnp_root->gp_seq, c) ||

		    (rnp != rnp_root &&

		     rnp_root->gpnum != rnp_root->completed)) {

		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {

			trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));

			goto unlock_out;

		}
@@ -1650,7 +1604,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, 
	}



	/* If GP already in progress, just leave, otherwise start one. */

	if (rnp_root->gpnum != rnp_root->completed) {

	if (rcu_gp_in_progress(rsp)) {

		trace_rcu_this_gp(rnp_root, rdp, c, TPS("Startedleafroot"));

		goto unlock_out;

	}
@@ -1675,7 +1629,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, 
 */

static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)

{

	unsigned long c = rnp->completed;

	unsigned long c = rnp->gp_seq;

	bool needmore;

	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
@@ -1703,14 +1657,14 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp) 
}



/*

 * If there is room, assign a ->completed number to any callbacks on

 * this CPU that have not already been assigned.  Also accelerate any

 * callbacks that were previously assigned a ->completed number that has

 * since proven to be too conservative, which can happen if callbacks get

 * assigned a ->completed number while RCU is idle, but with reference to

 * a non-root rcu_node structure.  This function is idempotent, so it does

 * not hurt to call it repeatedly.  Returns an flag saying that we should

 * awaken the RCU grace-period kthread.

 * If there is room, assign a ->gp_seq number to any callbacks on this

 * CPU that have not already been assigned.  Also accelerate any callbacks

 * that were previously assigned a ->gp_seq number that has since proven

 * to be too conservative, which can happen if callbacks get assigned a

 * ->gp_seq number while RCU is idle, but with reference to a non-root

 * rcu_node structure.  This function is idempotent, so it does not hurt

 * to call it repeatedly.  Returns an flag saying that we should awaken

 * the RCU grace-period kthread.

 *

 * The caller must hold rnp->lock with interrupts disabled.

 */
@@ -1736,7 +1690,7 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp, 
	 * accelerating callback invocation to an earlier grace-period

	 * number.

	 */

	c = rcu_cbs_completed(rsp, rnp);

	c = rcu_seq_snap(&rsp->gp_seq);

	if (rcu_segcblist_accelerate(&rdp->cblist, c))

		ret = rcu_start_this_gp(rnp, rdp, c);
@@ -1751,7 +1705,7 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp, 
/*

 * Move any callbacks whose grace period has completed to the

 * RCU_DONE_TAIL sublist, then compact the remaining sublists and

 * assign ->completed numbers to any callbacks in the RCU_NEXT_TAIL

 * assign ->gp_seq numbers to any callbacks in the RCU_NEXT_TAIL

 * sublist.  This function is idempotent, so it does not hurt to

 * invoke it repeatedly.  As long as it is not invoked -too- often...

 * Returns true if the RCU grace-period kthread needs to be awakened.
@@ -1768,10 +1722,10 @@ static bool rcu_advance_cbs(struct rcu_state *rsp, struct rcu_node *rnp, 
		return false;



	/*

	 * Find all callbacks whose ->completed numbers indicate that they

	 * Find all callbacks whose ->gp_seq numbers indicate that they

	 * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.

	 */

	rcu_segcblist_advance(&rdp->cblist, rnp->completed);

	rcu_segcblist_advance(&rdp->cblist, rnp->gp_seq);



	/* Classify any remaining callbacks. */

	return rcu_accelerate_cbs(rsp, rnp, rdp);
@@ -1889,6 +1843,8 @@ static bool rcu_gp_init(struct rcu_state *rsp) 
	smp_store_release(&rsp->gpnum, rsp->gpnum + 1);

	smp_mb(); /* Pairs with barriers in stall-warning code. */

	rcu_seq_start(&rsp->gp_seq);

	if (WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq))) /* Catch any ->completed/->gp_seq mismatches. */

		pr_info("%s ->completed: %#lx (%#lx) ->gp_seq %#lx (%#lx)\n", __func__, rnp->completed, (rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT, rnp->gp_seq, rcu_seq_ctr(rnp->gp_seq));

	trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));

	raw_spin_unlock_irq_rcu_node(rnp);

kernel/rcu/tree.h

+1 −1
Original line number Diff line number Diff line
@@ -174,7 +174,7 @@ struct rcu_node { 
#define need_future_gp_mask() \

	(ARRAY_SIZE(((struct rcu_node *)NULL)->need_future_gp) - 1)

#define need_future_gp_element(rnp, c) \

	((rnp)->need_future_gp[(c) & need_future_gp_mask()])

	((rnp)->need_future_gp[(c >> RCU_SEQ_CTR_SHIFT) & need_future_gp_mask()])

#define need_any_future_gp(rnp)						\

({									\

	int __i;							\

kernel/rcu/tree_plugin.h

+3 −3
Original line number Diff line number Diff line
@@ -2105,7 +2105,7 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp) 
	struct rcu_node *rnp = rdp->mynode;



	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	c = rcu_cbs_completed(rdp->rsp, rnp);

	c = rcu_seq_snap(&rdp->rsp->gp_seq);

	needwake = rcu_start_this_gp(rnp, rdp, c);

	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	if (needwake)
@@ -2118,8 +2118,8 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp) 
	trace_rcu_this_gp(rnp, rdp, c, TPS("StartWait"));

	for (;;) {

		swait_event_interruptible(

			rnp->nocb_gp_wq[c & 0x1],

			(d = ULONG_CMP_GE(READ_ONCE(rnp->completed), c)));

			rnp->nocb_gp_wq[rcu_seq_ctr(c) & 0x1],

			(d = rcu_seq_done(&rnp->gp_seq, c)));

		if (likely(d))

			break;

		WARN_ON(signal_pending(current));