[PATCH v3] 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
(*
 * bad condition: P0's first scan (SCAN1) saw P1's idx=0 LOCK count inc, though P1 saw flip.
 *
 * So basically, the ->po ordering on both P0 and P1 is enforced via ->ppo
 * (control deps) on both sides, and both P0 and P1 are interconnected by ->rf
 * relations. Combining the ->ppo with ->rf, a cycle is impossible.
 *)

{}

// 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    // Remove E and still passes.
                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) {         // Control dep
                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);
        }
}

exists (0:lock1=1 /\ 1:idx1=1)

This commit therefore removes memory barrier E, as memory barriers are not
free, and clarifies the old comment.

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>

---
v1->v2: Update changelog, keep old comments.
v2->v3: Moar changelog updates.


 kernel/rcu/srcutree.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 1c304fec89c0..0f9ba0f9fd12 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -983,15 +983,15 @@ 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 dependencies on both reader and updater side ensures 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 ensures 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).
+	 *
+	 * So no need for an smp_mb() before incrementing srcu_idx.
 	 */
-	smp_mb(); /* E */  /* Pairs with B and C. */
-
 	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 
 	/*
-- 
2.39.0.314.g84b9a713c41-goog