svn commit: r363654 - head/sys/vm

[Mark Johnston]

Author: markj
Date: Tue Jul 28 19:50:39 2020
New Revision: 363654
URL: https://svnweb.freebsd.org/changeset/base/363654

Log:
  vm_page_xbusy_claim(): Use atomics to update busy lock state.
  
  vm_page_xbusy_claim() could clobber the waiter bit.  For its original
  use, kernel memory pages, this was not a problem since nothing would
  ever block on the busy lock for such pages.  r363607 introduced a new
  use where this could in principle be a problem.
  
  Fix the problem by using atomic_cmpset to update the lock owner.  Since
  this macro is defined only for INVARIANTS kernels the extra overhead
  doesn't seem prohibitive.
  
  Reported by:	vangyzen
  Reviewed by:	alc, kib, vangyzen
  Sponsored by:	The FreeBSD Foundation
  Differential Revision:	https://reviews.freebsd.org/D25859

Modified:
  head/sys/vm/vm_page.h

Modified: head/sys/vm/vm_page.h
==============================================================================
--- head/sys/vm/vm_page.h	Tue Jul 28 19:35:24 2020	(r363653)
+++ head/sys/vm/vm_page.h	Tue Jul 28 19:50:39 2020	(r363654)
@@ -772,8 +772,13 @@ void vm_page_assert_pga_writeable(vm_page_t m, uint16_
 #define	VM_PAGE_ASSERT_PGA_WRITEABLE(m, bits)				\
 	vm_page_assert_pga_writeable(m, bits)
 #define	vm_page_xbusy_claim(m) do {					\
+	u_int _busy_lock;						\
+									\
 	vm_page_assert_xbusied_unchecked((m));				\
-	(m)->busy_lock = VPB_CURTHREAD_EXCLUSIVE;			\
+	do {								\
+		_busy_lock = atomic_load_int(&(m)->busy_lock);		\
+	} while (!atomic_cmpset_int(&(m)->busy_lock, _busy_lock,	\
+	    (_busy_lock & VPB_BIT_FLAGMASK) | VPB_CURTHREAD_EXCLUSIVE)); \
 } while (0)
 #else
 #define	VM_PAGE_OBJECT_BUSY_ASSERT(m)	(void)0