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Date:      Thu, 5 Oct 2017 13:01:18 +0000 (UTC)
From:      Mateusz Guzik <mjg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r324314 - head/sys/kern
Message-ID:  <201710051301.v95D1Iki056295@repo.freebsd.org>
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Author: mjg
Date: Thu Oct  5 13:01:18 2017
New Revision: 324314
URL: https://svnweb.freebsd.org/changeset/base/324314

Log:
  locks: partially tidy up waiting on readers
  
  spin first instant of instantly re-readoing and don't re-read after
  spinning is finished - the state is already known.
  
  Note the code is subject to significant changes later.
  
  MFC after:	1 week

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

Modified: head/sys/kern/kern_rwlock.c
==============================================================================
--- head/sys/kern/kern_rwlock.c	Thu Oct  5 12:50:03 2017	(r324313)
+++ head/sys/kern/kern_rwlock.c	Thu Oct  5 13:01:18 2017	(r324314)
@@ -489,12 +489,11 @@ __rw_rlock_hard(volatile uintptr_t *c, struct thread *
 			    "spinning", "lockname:\"%s\"",
 			    rw->lock_object.lo_name);
 			for (i = 0; i < rowner_loops; i++) {
+				cpu_spinwait();
 				v = RW_READ_VALUE(rw);
 				if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(td, v))
 					break;
-				cpu_spinwait();
 			}
-			v = RW_READ_VALUE(rw);
 #ifdef KDTRACE_HOOKS
 			lda.spin_cnt += rowner_loops - i;
 #endif
@@ -930,13 +929,13 @@ __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v, ui
 			    "spinning", "lockname:\"%s\"",
 			    rw->lock_object.lo_name);
 			for (i = 0; i < rowner_loops; i++) {
-				if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
-					break;
 				cpu_spinwait();
+				v = RW_READ_VALUE(rw);
+				if ((v & RW_LOCK_WRITE_SPINNER) == 0)
+					break;
 			}
 			KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 			    "running");
-			v = RW_READ_VALUE(rw);
 #ifdef KDTRACE_HOOKS
 			lda.spin_cnt += rowner_loops - i;
 #endif

Modified: head/sys/kern/kern_sx.c
==============================================================================
--- head/sys/kern/kern_sx.c	Thu Oct  5 12:50:03 2017	(r324313)
+++ head/sys/kern/kern_sx.c	Thu Oct  5 13:01:18 2017	(r324314)
@@ -605,18 +605,17 @@ _sx_xlock_hard(struct sx *sx, uintptr_t x, uintptr_t t
 						CTR4(KTR_LOCK,
 				    "%s: shared spinning on %p with %u and %u",
 						    __func__, sx, spintries, i);
-					x = sx->sx_lock;
+					cpu_spinwait();
+					x = SX_READ_VALUE(sx);
 					if ((x & SX_LOCK_SHARED) == 0 ||
 					    SX_SHARERS(x) == 0)
 						break;
-					cpu_spinwait();
+				}
 #ifdef KDTRACE_HOOKS
-					lda.spin_cnt++;
+				lda.spin_cnt += i;
 #endif
-				}
 				KTR_STATE0(KTR_SCHED, "thread",
 				    sched_tdname(curthread), "running");
-				x = SX_READ_VALUE(sx);
 				if (i != asx_loops)
 					continue;
 			}

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