svn commit: r307064 - head/sys/vm

[Konstantin Belousov]

Author: kib
Date: Tue Oct 11 18:09:37 2016
New Revision: 307064
URL: https://svnweb.freebsd.org/changeset/base/307064

Log:
  When downgrading exclusively busied page to shared-busy state, wakeup
  waiters.  Otherwise, owners of the shared-busy state are left blocked
  and might get into a deadlock.
  
  Note that the vm_page_busy_downgrade() function is not used in the
  tree right now.
  
  Reported and tested by:	pho (previous version)
  Reviewed by:	alc, markj
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week
  Differential revision:	https://reviews.freebsd.org/D8195

Modified:
  head/sys/vm/vm_page.c

Modified: head/sys/vm/vm_page.c
==============================================================================
--- head/sys/vm/vm_page.c	Tue Oct 11 17:47:54 2016	(r307063)
+++ head/sys/vm/vm_page.c	Tue Oct 11 18:09:37 2016	(r307064)
@@ -655,15 +655,26 @@ void
 vm_page_busy_downgrade(vm_page_t m)
 {
 	u_int x;
+	bool locked;
 
 	vm_page_assert_xbusied(m);
+	locked = mtx_owned(vm_page_lockptr(m));
 
 	for (;;) {
 		x = m->busy_lock;
 		x &= VPB_BIT_WAITERS;
+		if (x != 0 && !locked)
+			vm_page_lock(m);
 		if (atomic_cmpset_rel_int(&m->busy_lock,
-		    VPB_SINGLE_EXCLUSIVER | x, VPB_SHARERS_WORD(1) | x))
+		    VPB_SINGLE_EXCLUSIVER | x, VPB_SHARERS_WORD(1)))
 			break;
+		if (x != 0 && !locked)
+			vm_page_unlock(m);
+	}
+	if (x != 0) {
+		wakeup(m);
+		if (!locked)
+			vm_page_unlock(m);
 	}
 }