acpi/x86: ignore unspecified bit positions in the ACPI global lock field

The value in "new" is constructed from "old" such that all bits defined
as reserved by the ACPI spec[1] are left untouched. But if those bits
do not happen to be all zero, "new < 3" will not evaluate to true.

The firmware of the laptop(s) Medion MD63490 / Akoya P15648 comes with
garbage inside the "FACS" ACPI table. The starting value is
old=0x4944454d, therefore new=0x4944454e, which is >= 3. Mask off
the reserved bits.

[1] https://uefi.org/sites/default/files/resources/ACPI_6_2.pdf

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206553
Cc: All applicable <[email protected]>
Signed-off-by: Jan Engelhardt <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: jengelh

arch/x86/kernel/acpi/boot.c

@@ -1748,7 +1748,7 @@ int __acpi_acquire_global_lock(unsigned int *lock)
 		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
 		val = cmpxchg(lock, old, new);
 	} while (unlikely (val != old));
-	return (new < 3) ? -1 : 0;
+	return ((new & 0x3) < 3) ? -1 : 0;
 }
 
 int __acpi_release_global_lock(unsigned int *lock)