mm/rmap: use atomic_try_cmpxchg in set_tlb_ubc_flush_pending

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in set_tlb_ubc_flush_pending.  86 CMPXCHG instruction returns success in
ZF flag, so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails.

No functional change intended.

Link: https://lkml.kernel.org/r/20230227214228.3533299-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 8632e02..1ea2756 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -644,7 +644,7 @@ void try_to_unmap_flush_dirty(void)
 static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
 {
        struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
-       int batch, nbatch;
+       int batch;
 
        arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
        tlb_ubc->flush_required = true;
@@ -662,11 +662,8 @@ retry:
                 * overflow.  Reset `pending' and `flushed' to be 1 and 0 if
                 * `pending' becomes large.
                 */
-               nbatch = atomic_cmpxchg(&mm->tlb_flush_batched, batch, 1);
-               if (nbatch != batch) {
-                       batch = nbatch;
+               if (!atomic_try_cmpxchg(&mm->tlb_flush_batched, &batch, 1))
                        goto retry;
-               }
        } else {
                atomic_inc(&mm->tlb_flush_batched);
        }