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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue, 18 Oct 2016 10:33:28 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r307555 - stable/10/sys/vm
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Author: kib
Date: Tue Oct 18 10:33:28 2016
New Revision: 307555
URL: https://svnweb.freebsd.org/changeset/base/307555

Log:
  MFC r307064:
  When downgrading exclusively busied page to shared-busy state, wakeup
  waiters.

Modified:
  stable/10/sys/vm/vm_page.c
Directory Properties:
  stable/10/   (props changed)

Modified: stable/10/sys/vm/vm_page.c
==============================================================================
--- stable/10/sys/vm/vm_page.c	Tue Oct 18 10:20:58 2016	(r307554)
+++ stable/10/sys/vm/vm_page.c	Tue Oct 18 10:33:28 2016	(r307555)
@@ -515,15 +515,26 @@ 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);
 	}
 }