[PATCH] locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

[Uros Bizjak <ubizjak@xxxxxxxxx>]

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG.

Also, try_cmpxchg() implicitly assigns old *ptr value to "old"
when CMPXCHG fails. There is no need to explicitly assign
old *ptr value to the temporary, which can simplify the
surrounding source code.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
---
 arch/x86/xen/p2m.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 9bdc3b656b2c..99918beccd80 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -555,7 +555,6 @@ int xen_alloc_p2m_entry(unsigned long pfn)
                        /* Separately check the mid mfn level */
                        unsigned long missing_mfn;
                        unsigned long mid_mfn_mfn;
-                       unsigned long old_mfn;
 
                        mid_mfn = alloc_p2m_page();
                        if (!mid_mfn)
@@ -565,12 +564,12 @@ int xen_alloc_p2m_entry(unsigned long pfn)
 
                        missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
                        mid_mfn_mfn = virt_to_mfn(mid_mfn);
-                       old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
-                       if (old_mfn != missing_mfn) {
-                               free_p2m_page(mid_mfn);
-                               mid_mfn = mfn_to_virt(old_mfn);
-                       } else {
+                       /* try_cmpxchg() updates missing_mfn on failure. */
+                       if (try_cmpxchg(top_mfn_p, &missing_mfn, mid_mfn_mfn)) {
                                p2m_top_mfn_p[topidx] = mid_mfn;
+                       } else {
+                               free_p2m_page(mid_mfn);
+                               mid_mfn = mfn_to_virt(missing_mfn);
                        }
                }
        } else {
-- 
2.44.0