sched/membarrier: Fix redundant load of membarrier_state

On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
is not selected, sync_core_before_usermode() is a no-op.
In membarrier_mm_sync_core_before_usermode() the compiler does not
eliminate redundant branches and load of mm->membarrier_state
for this case as the atomic_read() cannot be optimized away.

Here's a snippet of the code generated for finish_task_switch() on powerpc
prior to this change:

  1b786c:   ld      r26,2624(r30)   # mm = rq->prev_mm;
  .......
  1b78c8:   cmpdi   cr7,r26,0
  1b78cc:   beq     cr7,1b78e4 <finish_task_switch+0xd0>
  1b78d0:   ld      r9,2312(r13)    # current
  1b78d4:   ld      r9,1888(r9)     # current->mm
  1b78d8:   cmpd    cr7,r26,r9
  1b78dc:   beq     cr7,1b7a70 <finish_task_switch+0x25c>
  1b78e0:   hwsync
  1b78e4:   cmplwi  cr7,r27,128
  .......
  1b7a70:   lwz     r9,176(r26)     # atomic_read(&mm->membarrier_state)
  1b7a74:   b       1b78e0 <finish_task_switch+0xcc>

This was found while analyzing "perf c2c" reports on kernels prior
to commit c1753fd ("mm: move mm_count into its own cache line")
where mm_count was false sharing with membarrier_state.

There is a minor improvement in the size of finish_task_switch().
The following are results from bloat-o-meter for ppc64le:

  GCC 7.5.0
  ---------
  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
  Function                                     old     new   delta
  finish_task_switch                           884     852     -32

  GCC 12.2.1
  ----------
  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
  Function                                     old     new   delta
  finish_task_switch.isra                      852     820     -32

  LLVM 17.0.6
  -----------
  add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
  Function                                     old     new   delta
  rt_mutex_schedule                            120     104     -16
  finish_task_switch                           792     772     -20

Results on aarch64:

  GCC 14.1.1
  ----------
  add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
  Function                                     old     new   delta
  get_nohz_timer_target                        352     356      +4
  e843419@0b02_0000d7e7_408                      8       -      -8
  e843419@01bb_000021d2_868                      8       -      -8
  finish_task_switch.isra                      592     548     -44

Signed-off-by: Nysal Jan K.A. <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Mathieu Desnoyers <[email protected]>
Reviewed-by: Michael Ellerman <[email protected]>
Reviewed-by: Segher Boessenkool <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Nysal Jan K.A.

include/linux/sched/mm.h

@@ -531,6 +531,13 @@ enum {
 
 static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 {
+	/*
+	 * The atomic_read() below prevents CSE. The following should
+	 * help the compiler generate more efficient code on architectures
+	 * where sync_core_before_usermode() is a no-op.
+	 */
+	if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
+		return;
 	if (current->mm != mm)
 		return;
 	if (likely(!(atomic_read(&mm->membarrier_state) &