Index: head/share/man/man9/rwlock.9
===================================================================
--- head/share/man/man9/rwlock.9	(revision 297393)
+++ head/share/man/man9/rwlock.9	(revision 297394)
@@ -1,334 +1,335 @@
 .\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
 .\" All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 13, 2014
+.Dd March 28, 2016
 .Dt RWLOCK 9
 .Os
 .Sh NAME
 .Nm rwlock ,
 .Nm rw_init ,
 .Nm rw_init_flags,
 .Nm rw_destroy ,
 .Nm rw_rlock ,
 .Nm rw_wlock ,
 .Nm rw_runlock ,
 .Nm rw_wunlock ,
 .Nm rw_unlock ,
 .Nm rw_try_rlock ,
 .Nm rw_try_upgrade ,
 .Nm rw_try_wlock ,
 .Nm rw_downgrade ,
 .Nm rw_sleep ,
 .Nm rw_initialized ,
 .Nm rw_wowned ,
 .Nm rw_assert ,
 .Nm RW_SYSINIT
 .Nd kernel reader/writer lock
 .Sh SYNOPSIS
 .In sys/param.h
 .In sys/lock.h
 .In sys/rwlock.h
 .Ft void
 .Fn rw_init "struct rwlock *rw" "const char *name"
 .Ft void
 .Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
 .Ft void
 .Fn rw_destroy "struct rwlock *rw"
 .Ft void
 .Fn rw_rlock "struct rwlock *rw"
 .Ft void
 .Fn rw_wlock "struct rwlock *rw"
 .Ft int
 .Fn rw_try_rlock "struct rwlock *rw"
 .Ft int
 .Fn rw_try_wlock "struct rwlock *rw"
 .Ft void
 .Fn rw_runlock "struct rwlock *rw"
 .Ft void
 .Fn rw_wunlock "struct rwlock *rw"
 .Ft void
 .Fn rw_unlock "struct rwlock *rw"
 .Ft int
 .Fn rw_try_upgrade "struct rwlock *rw"
 .Ft void
 .Fn rw_downgrade "struct rwlock *rw"
 .Ft int
 .Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
 .Ft int
 .Fn rw_initialized "const struct rwlock *rw"
 .Ft int
 .Fn rw_wowned "const struct rwlock *rw"
 .Pp
 .Cd "options INVARIANTS"
 .Cd "options INVARIANT_SUPPORT"
 .Ft void
 .Fn rw_assert "const struct rwlock *rw" "int what"
 .In sys/kernel.h
 .Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
 .Sh DESCRIPTION
 Reader/writer locks allow shared access to protected data by multiple threads,
 or exclusive access by a single thread.
 The threads with shared access are known as
 .Em readers
 since they only read the protected data.
 A thread with exclusive access is known as a
 .Em writer
 since it can modify protected data.
 .Pp
 Although reader/writer locks look very similar to
 .Xr sx 9
 locks, their usage pattern is different.
 Reader/writer locks can be treated as mutexes (see
 .Xr mutex 9 )
 with shared/exclusive semantics.
 Unlike
 .Xr sx 9 ,
 an
 .Nm
 can be locked while holding a non-spin mutex, and an
 .Nm
 cannot be held while sleeping.
 The
 .Nm
 locks have priority propagation like mutexes, but priority
 can be propagated only to writers.
 This limitation comes from the fact that readers
 are anonymous.
 Another important property is that readers can always recurse,
 and exclusive locks can be made recursive selectively.
 .Ss Macros and Functions
 .Bl -tag -width indent
 .It Fn rw_init "struct rwlock *rw" "const char *name"
 Initialize structure located at
 .Fa rw
 as reader/writer lock, described by name
 .Fa name .
 The description is used solely for debugging purposes.
 This function must be called before any other operations
 on the lock.
 .It Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
 Initialize the rw lock just like the
 .Fn rw_init
 function, but specifying a set of optional flags to alter the
 behaviour of
 .Fa rw ,
 through the
 .Fa opts
 argument.
 It contains one or more of the following flags:
 .Bl -tag -width ".Dv RW_NOPROFILE"
 .It Dv RW_DUPOK
 Witness should not log messages about duplicate locks being acquired.
 .It Dv RW_NOPROFILE
 Do not profile this lock.
 .It Dv RW_NOWITNESS
 Instruct
 .Xr witness 4
 to ignore this lock.
 .It Dv RW_QUIET
 Do not log any operations for this lock via
 .Xr ktr 4 .
 .It Dv RW_RECURSE
 Allow threads to recursively acquire exclusive locks for
 .Fa rw .
 .It Dv RW_NEW
 If the kernel has been compiled with
 .Cd "option INVARIANTS" ,
 .Fn rw_init_flags
 will assert that the
 .Fa rw
 has not been initialized multiple times without intervening calls to
 .Fn rw_destroy
 unless this option is specified.
 .El
 .It Fn rw_rlock "struct rwlock *rw"
 Lock
 .Fa rw
 as a reader.
 If any thread holds this lock exclusively, the current thread blocks,
 and its priority is propagated to the exclusive holder.
 The
 .Fn rw_rlock
 function can be called when the thread has already acquired reader
 access on
 .Fa rw .
 This is called
 .Dq "recursing on a lock" .
 .It Fn rw_wlock "struct rwlock *rw"
 Lock
 .Fa rw
 as a writer.
 If there are any shared owners of the lock, the current thread blocks.
 The
 .Fn rw_wlock
 function can be called recursively only if
 .Fa rw
 has been initialized with the
 .Dv RW_RECURSE
 option enabled.
 .It Fn rw_try_rlock "struct rwlock *rw"
 Try to lock
 .Fa rw
 as a reader.
 This function will return true if the operation succeeds, otherwise 0
 will be returned.
 .It Fn rw_try_wlock "struct rwlock *rw"
 Try to lock
 .Fa rw
 as a writer.
 This function will return true if the operation succeeds, otherwise 0
 will be returned.
 .It Fn rw_runlock "struct rwlock *rw"
 This function releases a shared lock previously acquired by
 .Fn rw_rlock .
 .It Fn rw_wunlock "struct rwlock *rw"
 This function releases an exclusive lock previously acquired by
 .Fn rw_wlock .
 .It Fn rw_unlock "struct rwlock *rw"
 This function releases a shared lock previously acquired by
 .Fn rw_rlock
 or an exclusive lock previously acquired by
 .Fn rw_wlock .
 .It Fn rw_try_upgrade "struct rwlock *rw"
 Attempt to upgrade a single shared lock to an exclusive lock.
 The current thread must hold a shared lock of
 .Fa rw .
 This will only succeed if the current thread holds the only shared lock on
 .Fa rw ,
 and it only holds a single shared lock.
 If the attempt succeeds
 .Fn rw_try_upgrade
 will return a non-zero value,
 and the current thread will hold an exclusive lock.
 If the attempt fails
 .Fn rw_try_upgrade
 will return zero,
 and the current thread will still hold a shared lock.
 .It Fn rw_downgrade "struct rwlock *rw"
 Convert an exclusive lock into a single shared lock.
 The current thread must hold an exclusive lock of
 .Fa rw .
 .It Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
 Atomically release
 .Fa rw
 while waiting for an event.
 For more details on the parameters to this function,
 see
 .Xr sleep 9 .
 .It Fn rw_initialized "const struct rwlock *rw"
 This function returns non-zero if
 .Fa rw
 has been initialized, and zero otherwise.
 .It Fn rw_destroy "struct rwlock *rw"
 This functions destroys a lock previously initialized with
 .Fn rw_init .
 The
 .Fa rw
 lock must be unlocked.
 .It Fn rw_wowned "const struct rwlock *rw"
 This function returns a non-zero value if the current thread owns an
 exclusive lock on
 .Fa rw .
 .It Fn rw_assert "const struct rwlock *rw" "int what"
 This function allows assertions specified in
 .Fa what
 to be made about
 .Fa rw .
 If the assertions are not true and the kernel is compiled
 with
 .Cd "options INVARIANTS"
 and
 .Cd "options INVARIANT_SUPPORT" ,
 the kernel will panic.
 Currently the following base assertions are supported:
 .Bl -tag -width ".Dv RA_UNLOCKED"
 .It Dv RA_LOCKED
 Assert that current thread holds either a shared or exclusive lock
 of
 .Fa rw .
 .It Dv RA_RLOCKED
 Assert that current thread holds a shared lock of
 .Fa rw .
 .It Dv RA_WLOCKED
 Assert that current thread holds an exclusive lock of
 .Fa rw .
 .It Dv RA_UNLOCKED
 Assert that current thread holds neither a shared nor exclusive lock of
 .Fa rw .
 .El
 .Pp
 In addition, one of the following optional flags may be specified with
 .Dv RA_LOCKED ,
 .Dv RA_RLOCKED ,
 or
 .Dv RA_WLOCKED :
 .Bl -tag -width ".Dv RA_NOTRECURSED"
 .It Dv RA_RECURSED
 Assert that the current thread holds a recursive lock of
 .Fa rw .
 .It Dv RA_NOTRECURSED
 Assert that the current thread does not hold a recursive lock of
 .Fa rw .
 .El
 .El
 .Sh SEE ALSO
 .Xr locking 9 ,
 .Xr mutex 9 ,
 .Xr panic 9 ,
 .Xr sema 9 ,
 .Xr sx 9
 .Sh HISTORY
 These
 functions appeared in
 .Fx 7.0 .
 .Sh AUTHORS
 .An -nosplit
 The
 .Nm
 facility was written by
 .An "John Baldwin" .
 This manual page was written by
 .An "Gleb Smirnoff" .
 .Sh BUGS
-If
+A kernel without
 .Dv WITNESS
-is not included in the kernel,
-then it is impossible to assert that the current thread does or does not
-hold a read lock.
-In the
-.Pf non- Dv WITNESS
-case, the
+cannot assert whether the current thread does or does not hold a read lock.
 .Dv RA_LOCKED
 and
 .Dv RA_RLOCKED
-assertions merely check that some thread holds a read lock.
+can only assert that
+.Em any
+thread holds a read lock.
+They cannot ensure that the current thread holds a read lock.
+Further,
+.Dv RA_UNLOCKED
+can only assert that the current thread does not hold a write lock.
 .Pp
 Reader/writer is a bit of an awkward name.
 An
 .Nm
 can also be called a
 .Dq Robert Watson
 lock if desired.
Index: head/share/man/man9/sx.9
===================================================================
--- head/share/man/man9/sx.9	(revision 297393)
+++ head/share/man/man9/sx.9	(revision 297394)
@@ -1,330 +1,335 @@
 .\"
 .\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>.  All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice(s), this list of conditions and the following disclaimer as
 .\"    the first lines of this file unmodified other than the possible
 .\"    addition of one or more copyright notices.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice(s), this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
 .\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 .\" DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
 .\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 .\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 .\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 .\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
 .\" DAMAGE.
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 16, 2016
+.Dd March 28, 2016
 .Dt SX 9
 .Os
 .Sh NAME
 .Nm sx ,
 .Nm sx_init ,
 .Nm sx_init_flags ,
 .Nm sx_destroy ,
 .Nm sx_slock ,
 .Nm sx_xlock ,
 .Nm sx_slock_sig ,
 .Nm sx_xlock_sig ,
 .Nm sx_try_slock ,
 .Nm sx_try_xlock ,
 .Nm sx_sunlock ,
 .Nm sx_xunlock ,
 .Nm sx_unlock ,
 .Nm sx_try_upgrade ,
 .Nm sx_downgrade ,
 .Nm sx_sleep ,
 .Nm sx_xholder ,
 .Nm sx_xlocked ,
 .Nm sx_assert ,
 .Nm SX_SYSINIT
 .Nd kernel shared/exclusive lock
 .Sh SYNOPSIS
 .In sys/param.h
 .In sys/lock.h
 .In sys/sx.h
 .Ft void
 .Fn sx_init "struct sx *sx" "const char *description"
 .Ft void
 .Fn sx_init_flags "struct sx *sx" "const char *description" "int opts"
 .Ft void
 .Fn sx_destroy "struct sx *sx"
 .Ft void
 .Fn sx_slock "struct sx *sx"
 .Ft void
 .Fn sx_xlock "struct sx *sx"
 .Ft int
 .Fn sx_slock_sig "struct sx *sx"
 .Ft int
 .Fn sx_xlock_sig "struct sx *sx"
 .Ft int
 .Fn sx_try_slock "struct sx *sx"
 .Ft int
 .Fn sx_try_xlock "struct sx *sx"
 .Ft void
 .Fn sx_sunlock "struct sx *sx"
 .Ft void
 .Fn sx_xunlock "struct sx *sx"
 .Ft void
 .Fn sx_unlock "struct sx *sx"
 .Ft int
 .Fn sx_try_upgrade "struct sx *sx"
 .Ft void
 .Fn sx_downgrade "struct sx *sx"
 .Ft int
 .Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
 .Ft "struct thread *"
 .Fn sx_xholder "struct sx *sx"
 .Ft int
 .Fn sx_xlocked "const struct sx *sx"
 .Pp
 .Cd "options INVARIANTS"
 .Cd "options INVARIANT_SUPPORT"
 .Ft void
 .Fn sx_assert "const struct sx *sx" "int what"
 .In sys/kernel.h
 .Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
 .Sh DESCRIPTION
 Shared/exclusive locks are used to protect data that are read far more often
 than they are written.
 Shared/exclusive locks do not implement priority propagation like mutexes and
 reader/writer locks to prevent priority inversions, so
 shared/exclusive locks should be used prudently.
 .Pp
 Shared/exclusive locks are created with either
 .Fn sx_init
 or
 .Fn sx_init_flags
 where
 .Fa sx
 is a pointer to space for a
 .Vt struct sx ,
 and
 .Fa description
 is a pointer to a null-terminated character string that describes the
 shared/exclusive lock.
 The
 .Fa opts
 argument to
 .Fn sx_init_flags
 specifies a set of optional flags to alter the behavior of
 .Fa sx .
 It contains one or more of the following flags:
 .Bl -tag -width SX_NOADAPTIVE
 .It Dv SX_NOADAPTIVE
 Disable adaptive spinning, rather than sleeping, for lock operations
 while an exclusive lock holder is executing on another CPU.
 Adaptive spinning is the default unless the kernel is compiled with
 .Cd "options NO_ADAPTIVE_SX" .
 .It Dv SX_DUPOK
 Witness should not log messages about duplicate locks being acquired.
 .It Dv SX_NOWITNESS
 Instruct
 .Xr witness 4
 to ignore this lock.
 .It Dv SX_NOPROFILE
 Do not profile this lock.
 .It Dv SX_RECURSE
 Allow threads to recursively acquire exclusive locks for
 .Fa sx .
 .It Dv SX_QUIET
 Do not log any operations for this lock via
 .Xr ktr 4 .
 .It Dv SX_NEW
 If the kernel has been compiled with
 .Cd "options INVARIANTS" ,
 .Fn sx_init
 will assert that the
 .Fa sx
 has not been initialized multiple times without intervening calls to
 .Fn sx_destroy
 unless this option is specified.
 .El
 .Pp
 Shared/exclusive locks are destroyed with
 .Fn sx_destroy .
 The lock
 .Fa sx
 must not be locked by any thread when it is destroyed.
 .Pp
 Threads acquire and release a shared lock by calling
 .Fn sx_slock ,
 .Fn sx_slock_sig
 or
 .Fn sx_try_slock
 and
 .Fn sx_sunlock
 or
 .Fn sx_unlock .
 Threads acquire and release an exclusive lock by calling
 .Fn sx_xlock ,
 .Fn sx_xlock_sig
 or
 .Fn sx_try_xlock
 and
 .Fn sx_xunlock
 or
 .Fn sx_unlock .
 A thread can attempt to upgrade a currently held shared lock to an exclusive
 lock by calling
 .Fn sx_try_upgrade .
 A thread that has an exclusive lock can downgrade it to a shared lock by
 calling
 .Fn sx_downgrade .
 .Pp
 .Fn sx_try_slock
 and
 .Fn sx_try_xlock
 will return 0 if the shared/exclusive lock cannot be acquired immediately;
 otherwise the shared/exclusive lock will be acquired and a non-zero value will
 be returned.
 .Pp
 .Fn sx_try_upgrade
 will return 0 if the shared lock cannot be upgraded to an exclusive lock
 immediately; otherwise the exclusive lock will be acquired and a non-zero value
 will be returned.
 .Pp
 .Fn sx_slock_sig
 and
 .Fn sx_xlock_sig
 do the same as their normal versions but performing an interruptible sleep.
 They return a non-zero value if the sleep has been interrupted by a signal
 or an interrupt, otherwise 0.
 .Pp
 A thread can atomically release a shared/exclusive lock while waiting for an
 event by calling
 .Fn sx_sleep .
 For more details on the parameters to this function,
 see
 .Xr sleep 9 .
 .Pp
 When compiled with
 .Cd "options INVARIANTS"
 and
 .Cd "options INVARIANT_SUPPORT" ,
 the
 .Fn sx_assert
 function tests
 .Fa sx
 for the assertions specified in
 .Fa what ,
 and panics if they are not met.
 One of the following assertions must be specified:
 .Bl -tag -width ".Dv SA_UNLOCKED"
 .It Dv SA_LOCKED
 Assert that the current thread has either a shared or an exclusive lock on the
 .Vt sx
 lock pointed to by the first argument.
 .It Dv SA_SLOCKED
 Assert that the current thread has a shared lock on the
 .Vt sx
 lock pointed to by
 the first argument.
 .It Dv SA_XLOCKED
 Assert that the current thread has an exclusive lock on the
 .Vt sx
 lock pointed to
 by the first argument.
 .It Dv SA_UNLOCKED
 Assert that the current thread has no lock on the
 .Vt sx
 lock pointed to
 by the first argument.
 .El
 .Pp
 In addition, one of the following optional assertions may be included with
 either an
 .Dv SA_LOCKED ,
 .Dv SA_SLOCKED ,
 or
 .Dv SA_XLOCKED
 assertion:
 .Bl -tag -width ".Dv SA_NOTRECURSED"
 .It Dv SA_RECURSED
 Assert that the current thread has a recursed lock on
 .Fa sx .
 .It Dv SA_NOTRECURSED
 Assert that the current thread does not have a recursed lock on
 .Fa sx .
 .El
 .Pp
 .Fn sx_xholder
 will return a pointer to the thread which currently holds an exclusive lock on
 .Fa sx .
 If no thread holds an exclusive lock on
 .Fa sx ,
 then
 .Dv NULL
 is returned instead.
 .Pp
 .Fn sx_xlocked
 will return non-zero if the current thread holds the exclusive lock;
 otherwise, it will return zero.
 .Pp
 For ease of programming,
 .Fn sx_unlock
 is provided as a macro frontend to the respective functions,
 .Fn sx_sunlock
 and
 .Fn sx_xunlock .
 Algorithms that are aware of what state the lock is in should use either
 of the two specific functions for a minor performance benefit.
 .Pp
 The
 .Fn SX_SYSINIT
 macro is used to generate a call to the
 .Fn sx_sysinit
 routine at system startup in order to initialize a given
 .Fa sx
 lock.
 The parameters are the same as
 .Fn sx_init
 but with an additional argument,
 .Fa name ,
 that is used in generating unique variable names for the related
 structures associated with the lock and the sysinit routine.
 .Pp
 A thread may not hold both a shared lock and an exclusive lock on the same
 lock simultaneously;
 attempting to do so will result in deadlock.
 .Sh CONTEXT
 A thread may hold a shared or exclusive lock on an
 .Nm
 lock while sleeping.
 As a result, an
 .Nm
 lock may not be acquired while holding a mutex.
 Otherwise, if one thread slept while holding an
 .Nm
 lock while another thread blocked on the same
 .Nm
 lock after acquiring a mutex, then the second thread would effectively
 end up sleeping while holding a mutex, which is not allowed.
 .Sh SEE ALSO
 .Xr lock 9 ,
 .Xr locking 9 ,
 .Xr mutex 9 ,
 .Xr panic 9 ,
 .Xr rwlock 9 ,
 .Xr sema 9
 .Sh BUGS
-In the
-.No non- Ns Dv WITNESS
-case, the
+A kernel without
+.Dv WITNESS
+cannot assert whether the current thread does or does not hold a shared lock.
 .Dv SA_LOCKED
 and
 .Dv SA_SLOCKED
-assertions merely check that some thread holds a shared lock.
-They do not ensure that the current thread holds a shared lock.
+can only assert that
+.Em any
+thread holds a shared lock.
+They cannot ensure that the current thread holds a shared lock.
+Further,
+.Dv SA_UNLOCKED
+can only assert that the current thread does not hold an exclusive lock.