locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

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@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20240405083335.507471-1-ubizjak@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

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 {