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Date:      Wed, 2 Sep 2009 17:33:51 +0000 (UTC)
From:      Attilio Rao <attilio@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r196772 - head/sys/kern
Message-ID:  <200909021733.n82HXpV6002870@svn.freebsd.org>
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Author: attilio
Date: Wed Sep  2 17:33:51 2009
New Revision: 196772
URL: http://svn.freebsd.org/changeset/base/196772

Log:
  Fix some bugs related to adaptive spinning:
  
  In the lockmgr support:
  - GIANT_RESTORE() is just called when the sleep finishes, so the current
    code can ends up into a giant unlock problem.  Fix it by appropriately
    call GIANT_RESTORE() when needed.  Note that this is not exactly ideal
    because for any interation of the adaptive spinning we drop and restore
    Giant, but the overhead should be not a factor.
  - In the lock held in exclusive mode case, after the adaptive spinning is
    brought to completition, we should just retry to acquire the lock
    instead to fallthrough. Fix that.
  - Fix a style nit
  
  In the sx support:
  - Call GIANT_SAVE() before than looping. This saves some overhead because
    in the current code GIANT_SAVE() is called several times.
  
  Tested by:	Giovanni Trematerra <giovanni dot trematerra at gmail dot com>

Modified:
  head/sys/kern/kern_lock.c
  head/sys/kern/kern_sx.c

Modified: head/sys/kern/kern_lock.c
==============================================================================
--- head/sys/kern/kern_lock.c	Wed Sep  2 16:47:10 2009	(r196771)
+++ head/sys/kern/kern_lock.c	Wed Sep  2 17:33:51 2009	(r196772)
@@ -467,7 +467,10 @@ __lockmgr_args(struct lock *lk, u_int fl
 			/*
 			 * If the owner is running on another CPU, spin until
 			 * the owner stops running or the state of the lock
-			 * changes.
+			 * changes.  We need a double-state handle here
+			 * because for a failed acquisition the lock can be
+			 * either held in exclusive mode or shared mode
+			 * (for the writer starvation avoidance technique).
 			 */
 			if (LK_CAN_ADAPT(lk, flags) && (x & LK_SHARE) == 0 &&
 			    LK_HOLDER(x) != LK_KERNPROC) {
@@ -491,8 +494,10 @@ __lockmgr_args(struct lock *lk, u_int fl
 				while (LK_HOLDER(lk->lk_lock) ==
 				    (uintptr_t)owner && TD_IS_RUNNING(owner))
 					cpu_spinwait();
+				GIANT_RESTORE();
+				continue;
 			} else if (LK_CAN_ADAPT(lk, flags) &&
-			    (x & LK_SHARE) !=0 && LK_SHARERS(x) &&
+			    (x & LK_SHARE) != 0 && LK_SHARERS(x) &&
 			    spintries < alk_retries) {
 				if (flags & LK_INTERLOCK) {
 					class->lc_unlock(ilk);
@@ -511,6 +516,7 @@ __lockmgr_args(struct lock *lk, u_int fl
 						break;
 					cpu_spinwait();
 				}
+				GIANT_RESTORE();
 				if (i != alk_loops)
 					continue;
 			}
@@ -704,6 +710,8 @@ __lockmgr_args(struct lock *lk, u_int fl
 				while (LK_HOLDER(lk->lk_lock) ==
 				    (uintptr_t)owner && TD_IS_RUNNING(owner))
 					cpu_spinwait();
+				GIANT_RESTORE();
+				continue;
 			} else if (LK_CAN_ADAPT(lk, flags) &&
 			    (x & LK_SHARE) != 0 && LK_SHARERS(x) &&
 			    spintries < alk_retries) {
@@ -727,6 +735,7 @@ __lockmgr_args(struct lock *lk, u_int fl
 						break;
 					cpu_spinwait();
 				}
+				GIANT_RESTORE();
 				if (i != alk_loops)
 					continue;
 			}

Modified: head/sys/kern/kern_sx.c
==============================================================================
--- head/sys/kern/kern_sx.c	Wed Sep  2 16:47:10 2009	(r196771)
+++ head/sys/kern/kern_sx.c	Wed Sep  2 17:33:51 2009	(r196772)
@@ -531,13 +531,13 @@ _sx_xlock_hard(struct sx *sx, uintptr_t 
 					continue;
 				}
 			} else if (SX_SHARERS(x) && spintries < asx_retries) {
+				GIANT_SAVE();
 				spintries++;
 				for (i = 0; i < asx_loops; i++) {
 					if (LOCK_LOG_TEST(&sx->lock_object, 0))
 						CTR4(KTR_LOCK,
 				    "%s: shared spinning on %p with %u and %u",
 						    __func__, sx, spintries, i);
-					GIANT_SAVE();
 					x = sx->sx_lock;
 					if ((x & SX_LOCK_SHARED) == 0 ||
 					    SX_SHARERS(x) == 0)

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