[PATCH 0/3] rcu/nocb updates v2

[PATCH 0/3] rcu/nocb updates v2

[Frederic Weisbecker]

Hi,

Here is a re-issue of some leftovers from a previous patchset.
Changes since last time:

1) Use rcu_get_jiffies_lazy_flush() instead of the raw variable (thanks Joel)
2) Further comment ordering expectations between grace period end and
   callbacks execution.
   
Thanks.

Frederic Weisbecker (3):
  rcu: Rename jiffies_till_flush to jiffies_lazy_flush
  rcu/nocb: Remove needless LOAD-ACQUIRE
  rcu/nocb: Remove needless full barrier after callback advancing

 kernel/rcu/rcu.h       |  8 ++++----
 kernel/rcu/rcuscale.c  |  6 +++---
 kernel/rcu/tree.c      |  6 ++++++
 kernel/rcu/tree_nocb.h | 26 ++++++++++++--------------
 4 files changed, 25 insertions(+), 21 deletions(-)

-- 
2.42.1

[PATCH 1/3] rcu: Rename jiffies_till_flush to jiffies_lazy_flush

[Frederic Weisbecker]

The variable name jiffies_till_flush is too generic and therefore:

* It may shadow a global variable
* It doesn't tell on what it operates

Make the name more precise, along with the related APIs.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/rcu.h       |  8 ++++----
 kernel/rcu/rcuscale.c  |  6 +++---
 kernel/rcu/tree_nocb.h | 22 +++++++++++-----------
 3 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index b531c33e9545..ff41423cd61c 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -541,11 +541,11 @@ enum rcutorture_type {
 };
 
 #if defined(CONFIG_RCU_LAZY)
-unsigned long rcu_lazy_get_jiffies_till_flush(void);
-void rcu_lazy_set_jiffies_till_flush(unsigned long j);
+unsigned long rcu_get_jiffies_lazy_flush(void);
+void rcu_set_jiffies_lazy_flush(unsigned long j);
 #else
-static inline unsigned long rcu_lazy_get_jiffies_till_flush(void) { return 0; }
-static inline void rcu_lazy_set_jiffies_till_flush(unsigned long j) { }
+static inline unsigned long rcu_get_jiffies_lazy_flush(void) { return 0; }
+static inline void rcu_set_jiffies_lazy_flush(unsigned long j) { }
 #endif
 
 #if defined(CONFIG_TREE_RCU)
diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index ffdb30495e3c..8db4fedaaa1e 100644
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -764,9 +764,9 @@ kfree_scale_init(void)
 
 	if (kfree_by_call_rcu) {
 		/* do a test to check the timeout. */
-		orig_jif = rcu_lazy_get_jiffies_till_flush();
+		orig_jif = rcu_get_jiffies_lazy_flush();
 
-		rcu_lazy_set_jiffies_till_flush(2 * HZ);
+		rcu_set_jiffies_lazy_flush(2 * HZ);
 		rcu_barrier();
 
 		jif_start = jiffies;
@@ -775,7 +775,7 @@ kfree_scale_init(void)
 
 		smp_cond_load_relaxed(&rcu_lazy_test1_cb_called, VAL == 1);
 
-		rcu_lazy_set_jiffies_till_flush(orig_jif);
+		rcu_set_jiffies_lazy_flush(orig_jif);
 
 		if (WARN_ON_ONCE(jiffies_at_lazy_cb - jif_start < 2 * HZ)) {
 			pr_alert("ERROR: call_rcu() CBs are not being lazy as expected!\n");
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 4efbf7333d4e..aecef51166c7 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -256,6 +256,7 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
 	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
 }
 
+#ifdef CONFIG_RCU_LAZY
 /*
  * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
  * can elapse before lazy callbacks are flushed. Lazy callbacks
@@ -264,21 +265,20 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
  * left unsubmitted to RCU after those many jiffies.
  */
 #define LAZY_FLUSH_JIFFIES (10 * HZ)
-static unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
+static unsigned long jiffies_lazy_flush = LAZY_FLUSH_JIFFIES;
 
-#ifdef CONFIG_RCU_LAZY
 // To be called only from test code.
-void rcu_lazy_set_jiffies_till_flush(unsigned long jif)
+void rcu_set_jiffies_lazy_flush(unsigned long jif)
 {
-	jiffies_till_flush = jif;
+	jiffies_lazy_flush = jif;
 }
-EXPORT_SYMBOL(rcu_lazy_set_jiffies_till_flush);
+EXPORT_SYMBOL(rcu_set_jiffies_lazy_flush);
 
-unsigned long rcu_lazy_get_jiffies_till_flush(void)
+unsigned long rcu_get_jiffies_lazy_flush(void)
 {
-	return jiffies_till_flush;
+	return jiffies_lazy_flush;
 }
-EXPORT_SYMBOL(rcu_lazy_get_jiffies_till_flush);
+EXPORT_SYMBOL(rcu_get_jiffies_lazy_flush);
 #endif
 
 /*
@@ -299,7 +299,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 	 */
 	if (waketype == RCU_NOCB_WAKE_LAZY &&
 	    rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
-		mod_timer(&rdp_gp->nocb_timer, jiffies + jiffies_till_flush);
+		mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
 		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
 	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
 		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
@@ -482,7 +482,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	// flush ->nocb_bypass to ->cblist.
 	if ((ncbs && !bypass_is_lazy && j != READ_ONCE(rdp->nocb_bypass_first)) ||
 	    (ncbs &&  bypass_is_lazy &&
-	     (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + jiffies_till_flush))) ||
+	     (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + rcu_get_jiffies_lazy_flush()))) ||
 	    ncbs >= qhimark) {
 		rcu_nocb_lock(rdp);
 		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
@@ -723,7 +723,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		lazy_ncbs = READ_ONCE(rdp->lazy_len);
 
 		if (bypass_ncbs && (lazy_ncbs == bypass_ncbs) &&
-		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + jiffies_till_flush) ||
+		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + rcu_get_jiffies_lazy_flush()) ||
 		     bypass_ncbs > 2 * qhimark)) {
 			flush_bypass = true;
 		} else if (bypass_ncbs && (lazy_ncbs != bypass_ncbs) &&
-- 
2.42.1

[PATCH 2/3] rcu/nocb: Remove needless LOAD-ACQUIRE

[Frederic Weisbecker]

The LOAD-ACQUIRE access performed on rdp->nocb_cb_sleep advertizes
ordering callback execution against grace period completion. However
this is contradicted by the following:

* This LOAD-ACQUIRE doesn't pair with anything. The only counterpart
  barrier that can be found is the smp_mb() placed after callbacks
  advancing in nocb_gp_wait(). However the barrier is placed _after_
  ->nocb_cb_sleep write.

* Callbacks can be concurrently advanced between the LOAD-ACQUIRE on
  ->nocb_cb_sleep and the call to rcu_segcblist_extract_done_cbs() in
  rcu_do_batch(), making any ordering based on ->nocb_cb_sleep broken.

* Both rcu_segcblist_extract_done_cbs() and rcu_advance_cbs() are called
  under the nocb_lock, the latter hereby providing already the desired
  ACQUIRE semantics.

Therefore it is safe to access ->nocb_cb_sleep with a simple compiler
barrier.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree_nocb.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index aecef51166c7..eb27878d46f1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -933,8 +933,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
 						    nocb_cb_wait_cond(rdp));
 
-		// VVV Ensure CB invocation follows _sleep test.
-		if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
+		if (READ_ONCE(rdp->nocb_cb_sleep)) {
 			WARN_ON(signal_pending(current));
 			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
 		}
-- 
2.42.1

[PATCH 3/3] rcu/nocb: Remove needless full barrier after callback advancing

[Frederic Weisbecker]

A full barrier is issued from nocb_gp_wait() upon callbacks advancing
to order grace period completion with callbacks execution.

However these two events are already ordered by the
smp_mb__after_unlock_lock() barrier within the call to
raw_spin_lock_rcu_node() that is necessary for callbacks advancing to
happen.

The following litmus test shows the kind of guarantee that this barrier
provides:

	C smp_mb__after_unlock_lock

	{}

	// rcu_gp_cleanup()
	P0(spinlock_t *rnp_lock, int *gpnum)
	{
		// Grace period cleanup increase gp sequence number
		spin_lock(rnp_lock);
		WRITE_ONCE(*gpnum, 1);
		spin_unlock(rnp_lock);
	}

	// nocb_gp_wait()
	P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready)
	{
		int r1;

		// Call rcu_advance_cbs() from nocb_gp_wait()
		spin_lock(nocb_lock);
		spin_lock(rnp_lock);
		smp_mb__after_unlock_lock();
		r1 = READ_ONCE(*gpnum);
		WRITE_ONCE(*cb_ready, 1);
		spin_unlock(rnp_lock);
		spin_unlock(nocb_lock);
	}

	// nocb_cb_wait()
	P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed)
	{
		int r2;

		// rcu_do_batch() -> rcu_segcblist_extract_done_cbs()
		spin_lock(nocb_lock);
		r2 = READ_ONCE(*cb_ready);
		spin_unlock(nocb_lock);

		// Actual callback execution
		WRITE_ONCE(*cb_executed, 1);
	}

	P3(int *cb_executed, int *gpnum)
	{
		int r3;

		WRITE_ONCE(*cb_executed, 2);
		smp_mb();
		r3 = READ_ONCE(*gpnum);
	}

	exists (1:r1=1 /\ 2:r2=1 /\ cb_executed=2 /\ 3:r3=0) (* Bad outcome. *)

Here the bad outcome only occurs if the smp_mb__after_unlock_lock() is
removed. This barrier orders the grace period completion against
callbacks advancing and even later callbacks invocation, thanks to the
opportunistic propagation via the ->nocb_lock to nocb_cb_wait().

Therefore the smp_mb() placed after callbacks advancing can be safely
removed.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree.c      | 6 ++++++
 kernel/rcu/tree_nocb.h | 1 -
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3ac3c846105f..c557302fc4f5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2113,6 +2113,12 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	 * Extract the list of ready callbacks, disabling IRQs to prevent
 	 * races with call_rcu() from interrupt handlers.  Leave the
 	 * callback counts, as rcu_barrier() needs to be conservative.
+	 *
+	 * Callbacks execution is fully ordered against preceding grace period
+	 * completion (materialized by rnp->gp_seq update) thanks to the
+	 * smp_mb__after_unlock_lock() upon node locking required for callbacks
+	 * advancing. In NOCB mode this ordering is then further relayed through
+	 * the nocb locking that protects both callbacks advancing and extraction.
 	 */
 	rcu_nocb_lock_irqsave(rdp, flags);
 	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index eb27878d46f1..d82f96a66600 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -779,7 +779,6 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
 			needwake = rdp->nocb_cb_sleep;
 			WRITE_ONCE(rdp->nocb_cb_sleep, false);
-			smp_mb(); /* CB invocation -after- GP end. */
 		} else {
 			needwake = false;
 		}
-- 
2.42.1

Re: [PATCH 0/3] rcu/nocb updates v2

[Paul E. McKenney]

On Wed, Nov 15, 2023 at 02:11:25PM -0500, Frederic Weisbecker wrote:
> Hi,
> 
> Here is a re-issue of some leftovers from a previous patchset.
> Changes since last time:
> 
> 1) Use rcu_get_jiffies_lazy_flush() instead of the raw variable (thanks Joel)
> 2) Further comment ordering expectations between grace period end and
>    callbacks execution.

Hearing no objections, I have queued these for further review and testing.

							Thanx, Paul

> Thanks.
> 
> Frederic Weisbecker (3):
>   rcu: Rename jiffies_till_flush to jiffies_lazy_flush
>   rcu/nocb: Remove needless LOAD-ACQUIRE
>   rcu/nocb: Remove needless full barrier after callback advancing
> 
>  kernel/rcu/rcu.h       |  8 ++++----
>  kernel/rcu/rcuscale.c  |  6 +++---
>  kernel/rcu/tree.c      |  6 ++++++
>  kernel/rcu/tree_nocb.h | 26 ++++++++++++--------------
>  4 files changed, 25 insertions(+), 21 deletions(-)
> 
> -- 
> 2.42.1
>