rcu: Remove fast check path from __synchronize_srcu()

The fastpath in __synchronize_srcu() is designed to handle cases where
there are a large number of concurrent calls for the same srcu_struct
structure.  However, the Linux kernel currently does not use SRCU in
this manner, so remove the fastpath checks for simplicity.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 1 insertions, 24 deletions

diff --git a/kernel/srcu.c b/kernel/srcu.c
index 84c9b97dc3d9..17e95bcc901c 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -308,7 +308,7 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
  */
 static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
 {
-        int idx;
+        int idx = 0;
 
         rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
                            !lock_is_held(&rcu_bh_lock_map) &&
@@ -316,32 +316,9 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
                            !lock_is_held(&rcu_sched_lock_map),
                            "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
 
-        smp_mb();  /* Ensure prior action happens before grace period. */
-        idx = ACCESS_ONCE(sp->completed);
-        smp_mb();  /* Access to ->completed before lock acquisition. */
         mutex_lock(&sp->mutex);
 
         /*
-         * Check to see if someone else did the work for us while we were
-         * waiting to acquire the lock.  We need -three- advances of
-         * the counter, not just one.  If there was but one, we might have
-         * shown up -after- our helper's first synchronize_sched(), thus
-         * having failed to prevent CPU-reordering races with concurrent
-         * srcu_read_unlock()s on other CPUs (see comment below).  If there
-         * was only two, we are guaranteed to have waited through only one
-         * full index-flip phase.  So we either (1) wait for three or
-         * (2) supply the additional ones we need.
-         */
-
-        if (sp->completed == idx + 2)
-                idx = 1;
-        else if (sp->completed == idx + 3) {
-                mutex_unlock(&sp->mutex);
-                return;
-        } else
-                idx = 0;
-
-        /*
          * If there were no helpers, then we need to do two flips of
          * the index.  The first flip is required if there are any
          * outstanding SRCU readers even if there are no new readers