mce: Stop using array-index-based RCU primitives

Because mce is arch-specific x86 code, there is little or no
performance benefit of using rcu_dereference_index_check() over using
smp_load_acquire().  It also turns out that mce is the only place that
array-index-based RCU is used, and it would be convenient to drop
this portion of the RCU API.

This patch therefore changes rcu_dereference_index_check() uses to
smp_load_acquire(), but keeping the lockdep diagnostics, and also
changes rcu_access_index() uses to READ_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: linux-edac@vger.kernel.org
Cc: Tony Luck <tony.luck@intel.com>
Acked-by: Borislav Petkov <bp@suse.de>

1 files changed, 7 insertions, 4 deletions

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index e535533d5ab8..1fd04a02d90a 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -53,9 +53,12 @@
 static DEFINE_MUTEX(mce_chrdev_read_mutex);
 
 #define rcu_dereference_check_mce(p) \
-        rcu_dereference_index_check((p), \
-                              rcu_read_lock_sched_held() || \
-                              lockdep_is_held(&mce_chrdev_read_mutex))
+({ \
+        rcu_lockdep_assert(rcu_read_lock_sched_held() || \
+                           lockdep_is_held(&mce_chrdev_read_mutex), \
+                           "suspicious rcu_dereference_check_mce() usage"); \
+        smp_load_acquire(&(p)); \
+})
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/mce.h>
@@ -1884,7 +1887,7 @@ out:
 static unsigned int mce_chrdev_poll(struct file *file, poll_table *wait)
 {
         poll_wait(file, &mce_chrdev_wait, wait);
-        if (rcu_access_index(mcelog.next))
+        if (READ_ONCE(mcelog.next))
                 return POLLIN | POLLRDNORM;
         if (!mce_apei_read_done && apei_check_mce())
                 return POLLIN | POLLRDNORM;