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Date:      Sun, 4 Mar 2018 22:01:23 +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: r330418 - head/sys/kern
Message-ID:  <201803042201.w24M1N0U000730@repo.freebsd.org>
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Author: mjg
Date: Sun Mar  4 22:01:23 2018
New Revision: 330418
URL: https://svnweb.freebsd.org/changeset/base/330418

Log:
  mtx: tidy up recursion handling in thread lock
  
  Normally after grabbing the lock it has to be verified we got the right one
  to begin with. However, if we are recursing, it must not change thus the
  check can be avoided. In particular this avoids a lock read for non-recursing
  case which found out the lock was changed.
  
  While here avoid an irq trip of this happens.
  
  Tested by:	pho (previous version)

Modified:
  head/sys/kern/kern_mutex.c

Modified: head/sys/kern/kern_mutex.c
==============================================================================
--- head/sys/kern/kern_mutex.c	Sun Mar  4 21:58:55 2018	(r330417)
+++ head/sys/kern/kern_mutex.c	Sun Mar  4 22:01:23 2018	(r330418)
@@ -829,10 +829,8 @@ _thread_lock(struct thread *td)
 		WITNESS_LOCK(&m->lock_object, LOP_EXCLUSIVE, file, line);
 		return;
 	}
-	if (m->mtx_recurse != 0)
-		m->mtx_recurse--;
-	else
-		_mtx_release_lock_quick(m);
+	MPASS(m->mtx_recurse == 0);
+	_mtx_release_lock_quick(m);
 slowpath_unlocked:
 	spinlock_exit();
 slowpath_noirq:
@@ -886,9 +884,10 @@ thread_lock_flags_(struct thread *td, int opts, const 
 	if (__predict_false(doing_lockprof))
 		spin_time -= lockstat_nsecs(&td->td_lock->lock_object);
 #endif
+	spinlock_enter();
+
 	for (;;) {
 retry:
-		spinlock_enter();
 		m = td->td_lock;
 		thread_lock_validate(m, opts, file, line);
 		v = MTX_READ_VALUE(m);
@@ -900,6 +899,7 @@ retry:
 			}
 			if (v == tid) {
 				m->mtx_recurse++;
+				MPASS(m == td->td_lock);
 				break;
 			}
 			lock_profile_obtain_lock_failed(&m->lock_object,
@@ -912,15 +912,18 @@ retry:
 				} else {
 					_mtx_lock_indefinite_check(m, &lda);
 				}
-				if (m != td->td_lock)
+				if (m != td->td_lock) {
+					spinlock_enter();
 					goto retry;
+				}
 				v = MTX_READ_VALUE(m);
 			} while (v != MTX_UNOWNED);
 			spinlock_enter();
 		}
 		if (m == td->td_lock)
 			break;
-		__mtx_unlock_spin(m);	/* does spinlock_exit() */
+		MPASS(m->mtx_recurse == 0);
+		_mtx_release_lock_quick(m);
 	}
 	LOCK_LOG_LOCK("LOCK", &m->lock_object, opts, m->mtx_recurse, file,
 	    line);

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