[PATCH] srcu: Remove memory barrier "E" as it does not do anything

[Joel Fernandes (Google)]

During a flip, we have a full memory barrier before srcu_idx is incremented.

The idea is we intend to order the first phase scan's read of lock
counters with the flipping of the index.

However, such ordering is already enforced because of the
control-dependency between the 2 scans. We would be flipping the index
only if lock and unlock counts matched.

But such match will not happen if there was a pending reader before the flip
in the first place (observation courtesy Mathieu Desnoyers).

The litmus test below shows this:
(test courtesy Frederic Weisbecker, Changes for ctrldep by Boqun/me):

C srcu

{}

// updater
P0(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1)
{
        int lock1;
        int unlock1;
        int lock0;
        int unlock0;

        // SCAN1
        unlock1 = READ_ONCE(*UNLOCK1);
        smp_mb(); // A
        lock1 = READ_ONCE(*LOCK1);

        // FLIP
        if (lock1 == unlock1) {         // Control dep
                smp_mb(); // E
                WRITE_ONCE(*IDX, 1);
                smp_mb(); // D

                // SCAN2
                unlock0 = READ_ONCE(*UNLOCK0);
                smp_mb(); // A
                lock0 = READ_ONCE(*LOCK0);
        }
}

// reader
P1(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1)
{
        int tmp;
        int idx1;
        int idx2;

        // 1st reader
        idx1 = READ_ONCE(*IDX);
        if (idx1 == 0) {
                tmp = READ_ONCE(*LOCK0);
                WRITE_ONCE(*LOCK0, tmp + 1);
                smp_mb(); /* B and C */
                tmp = READ_ONCE(*UNLOCK0);
                WRITE_ONCE(*UNLOCK0, tmp + 1);
        } else {
                tmp = READ_ONCE(*LOCK1);
                WRITE_ONCE(*LOCK1, tmp + 1);
                smp_mb(); /* B and C */
                tmp = READ_ONCE(*UNLOCK1);
                WRITE_ONCE(*UNLOCK1, tmp + 1);
        }

        // second reader
        idx2 = READ_ONCE(*IDX);
        if (idx2 == 0) {
                tmp = READ_ONCE(*LOCK0);
                WRITE_ONCE(*LOCK0, tmp + 1);
                smp_mb(); /* B and C */
                tmp = READ_ONCE(*UNLOCK0);
                WRITE_ONCE(*UNLOCK0, tmp + 1);
        } else {
                tmp = READ_ONCE(*LOCK1);
                WRITE_ONCE(*LOCK1, tmp + 1);
                smp_mb(); /* B and C */
                tmp = READ_ONCE(*UNLOCK1);
                WRITE_ONCE(*UNLOCK1, tmp + 1);
        }
}

The following bad condition will not occur even if memory barrier E
is dropped:

(* bad condition: SCAN1 saw lock count changes though 1st reader saw flip *)
 exists (0:lock1=1 /\ 1:idx1=1 /\ 1:idx2=1)

Co-developed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Co-developed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 1c304fec89c0..d1368d64fdba 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -983,15 +983,9 @@ static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
 static void srcu_flip(struct srcu_struct *ssp)
 {
 	/*
-	 * Ensure that if this updater saw a given reader's increment
-	 * from __srcu_read_lock(), that reader was using an old value
-	 * of ->srcu_idx.  Also ensure that if a given reader sees the
-	 * new value of ->srcu_idx, this updater's earlier scans cannot
-	 * have seen that reader's increments (which is OK, because this
-	 * grace period need not wait on that reader).
+	 * Control dependency locks==unlocks ensures that if a reader sees the
+	 * flip, previous scan would only see before-flip reader lock counts.
 	 */
-	smp_mb(); /* E */  /* Pairs with B and C. */
-
 	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 
 	/*
-- 
2.39.0.314.g84b9a713c41-goog