diff --git a/lib/libthr/libthr.3 b/lib/libthr/libthr.3
--- a/lib/libthr/libthr.3
+++ b/lib/libthr/libthr.3
@@ -1,5 +1,5 @@
 .\" Copyright (c) 2005 Robert N. M. Watson
-.\" Copyright (c) 2014,2015 The FreeBSD Foundation, Inc.
+.\" Copyright (c) 2014,2015,2021 The FreeBSD Foundation, Inc.
 .\" All rights reserved.
 .\"
 .\" Part of this documentation was written by
@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 5, 2020
+.Dd October 1, 2021
 .Dt LIBTHR 3
 .Os
 .Sh NAME
@@ -259,6 +259,63 @@
 This should be taken into account when interpreting
 .Xr ktrace 1
 logs.
+.Sh PROCESS-SHARED SYNCHRONIZATION OBJECTS
+In the
+.Li libthr
+implementation,
+user-visible types for all synchronization objects are pointers to the
+internal structures, allocated either at the corresponding
+.Fn pthread_<objtype>_init
+method call, or implicitly on first use, when a static initializer
+was specified.
+Initial implementation of the process-private locking object used this
+model with internal allocation, and addition of the process-shared objects
+was done in a way that did not break application binary interface.
+.Pp
+For process-private objects, the internal structure is allocated using
+either
+.Xr malloc 3
+or, for
+.Xr pthread_mutex_init 3 ,
+an internal memory allocator implemented in
+.Nm .
+The internal allocator for mutexes is used to avoid bootstrap issues
+with many
+.Xr malloc 3
+implementations which need working mutexes to function.
+Same allocator is used for thread-specific data, see
+.Xr pthread_setspecific 3 ,
+for the same reason.
+.Pp
+For process-shared objects, the internal structure is created by first
+allocating shared memory segment using
+.Xr _umtx_op 2
+operation
+.Dv UMTX_OP_SHM ,
+and then mapping it into process address space with
+.Xr mmap 2
+with
+.Dv MAP_SHARED
+flag.
+The POSIX standard requires that:
+.Bd -literal
+only the process-shared synchronization object itself can be used for
+performing synchronization.  It need not be referenced at the address
+used to initalize it (that is, another mapping of the same object can
+be used).
+.Ed
+.Pp
+With this implementation, the process-shared objects require initialization
+in each process that use them.
+In particular, if you map the shared memory containing the user portion of
+the process-shared object already initialized in different process, locking
+functions do not work on it.
+.Pp
+Another broken case is a forked child creating the object in memory shared
+with the parent, which cannot be used from parent.
+Note that processes should not use non-async-signal safe functions after
+.Xr fork 2
+anyway.
 .Sh SEE ALSO
 .Xr ktrace 1 ,
 .Xr ld-elf.so.1 1 ,
diff --git a/share/man/man3/Makefile b/share/man/man3/Makefile
--- a/share/man/man3/Makefile
+++ b/share/man/man3/Makefile
@@ -481,11 +481,13 @@
 PTHREAD_MLINKS+=pthread_mutexattr.3 pthread_mutexattr_destroy.3 \
 		pthread_mutexattr.3 pthread_mutexattr_getprioceiling.3 \
 		pthread_mutexattr.3 pthread_mutexattr_getprotocol.3 \
+		pthread_mutexattr.3 pthread_mutexattr_getpshared.3 \
 		pthread_mutexattr.3 pthread_mutexattr_getrobust.3 \
 		pthread_mutexattr.3 pthread_mutexattr_gettype.3 \
 		pthread_mutexattr.3 pthread_mutexattr_init.3 \
 		pthread_mutexattr.3 pthread_mutexattr_setprioceiling.3 \
 		pthread_mutexattr.3 pthread_mutexattr_setprotocol.3 \
+		pthread_mutexattr.3 pthread_mutexattr_setpshared.3 \
 		pthread_mutexattr.3 pthread_mutexattr_setrobust.3 \
 		pthread_mutexattr.3 pthread_mutexattr_settype.3
 PTHREAD_MLINKS+=pthread_mutexattr_getkind_np.3 pthread_mutexattr_setkind_np.3
diff --git a/share/man/man3/pthread_barrierattr.3 b/share/man/man3/pthread_barrierattr.3
--- a/share/man/man3/pthread_barrierattr.3
+++ b/share/man/man3/pthread_barrierattr.3
@@ -38,11 +38,15 @@
 .Ft int
 .Fn pthread_barrierattr_destroy "pthread_barrierattr_t *attr"
 .Ft int
-.Fn pthread_barrierattr_getpshared "const pthread_barrierattr_t *restrict attr" "int *restrict pshared"
+.Fo pthread_barrierattr_getpshared
+.Fa "const pthread_barrierattr_t *restrict attr" "int *restrict pshared"
+.Fc
 .Ft int
 .Fn pthread_barrierattr_init "pthread_barrierattr_t *attr"
 .Ft int
-.Fn pthread_barrierattr_setpshared "pthread_barrierattr_t *attr" "int pshared"
+.Fo pthread_barrierattr_setpshared
+.Fa "pthread_barrierattr_t *attr" "int pshared"
+.Fc
 .Sh DESCRIPTION
 The
 .Fn pthread_barrierattr_init
diff --git a/share/man/man3/pthread_condattr.3 b/share/man/man3/pthread_condattr.3
--- a/share/man/man3/pthread_condattr.3
+++ b/share/man/man3/pthread_condattr.3
@@ -46,11 +46,15 @@
 .Ft int
 .Fn pthread_condattr_destroy "pthread_condattr_t *attr"
 .Ft int
-.Fn pthread_condattr_getclock "pthread_condattr_t * restrict attr" "clockid_t * restrict clock_id"
+.Fo pthread_condattr_getclock
+.Fa "pthread_condattr_t * restrict attr" "clockid_t * restrict clock_id"
+.Fc
 .Ft int
 .Fn pthread_condattr_setclock "pthread_condattr_t *attr" "clockid_t clock_id"
 .Ft int
-.Fn pthread_condattr_getpshared "pthread_condattr_t * restrict attr" "int * restrict pshared"
+.Fo pthread_condattr_getpshared
+.Fa "pthread_condattr_t * restrict attr" "int * restrict pshared"
+.Fc 
 .Ft int
 .Fn pthread_condattr_setpshared "pthread_condattr_t *attr" "int pshared"
 .Sh DESCRIPTION
diff --git a/share/man/man3/pthread_mutexattr.3 b/share/man/man3/pthread_mutexattr.3
--- a/share/man/man3/pthread_mutexattr.3
+++ b/share/man/man3/pthread_mutexattr.3
@@ -1,6 +1,11 @@
 .\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>.
+.\" Copyright (c) 2021 The FreeBSD Foundation, Inc.
 .\" All rights reserved.
 .\"
+.\" Part of this documentation was written by
+.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
+.\" from the FreeBSD Foundation.
+.\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
@@ -26,7 +31,7 @@
 .\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
 .\" $FreeBSD$
-.Dd August 17, 2018
+.Dd October 1, 2021
 .Dt PTHREAD_MUTEXATTR 3
 .Os
 .Sh NAME
@@ -36,6 +41,8 @@
 .Nm pthread_mutexattr_getprioceiling ,
 .Nm pthread_mutexattr_setprotocol ,
 .Nm pthread_mutexattr_getprotocol ,
+.Nm pthread_mutexattr_setpshared ,
+.Nm pthread_mutexattr_getpshared ,
 .Nm pthread_mutexattr_setrobust ,
 .Nm pthread_mutexattr_getrobust ,
 .Nm pthread_mutexattr_settype ,
@@ -50,13 +57,27 @@
 .Ft int
 .Fn pthread_mutexattr_destroy "pthread_mutexattr_t *attr"
 .Ft int
-.Fn pthread_mutexattr_setprioceiling "pthread_mutexattr_t *attr" "int prioceiling"
+.Fo pthread_mutexattr_setprioceiling
+.Fa "pthread_mutexattr_t *attr" "int prioceiling"
+.Fc
 .Ft int
-.Fn pthread_mutexattr_getprioceiling "const pthread_mutexattr_t *attr" "int *prioceiling"
+.Fo pthread_mutexattr_getprioceiling
+.Fa "const pthread_mutexattr_t *attr" "int *prioceiling"
+.Fc
 .Ft int
 .Fn pthread_mutexattr_setprotocol "pthread_mutexattr_t *attr" "int protocol"
 .Ft int
-.Fn pthread_mutexattr_getprotocol "const pthread_mutexattr_t *restrict attr" "int *restrict protocol"
+.Fo pthread_mutexattr_getprotocol
+.Fa "const pthread_mutexattr_t *restrict attr" "int *restrict protocol"
+.Fc
+.Ft int
+.Fo pthread_mutexattr_setpshared
+.Fa "pthread_mutexattr_t *attr" "int shared"
+.Fc
+.Ft int
+.Fo pthread_mutexattr_getpshared
+.Fa "const pthread_mutexattr_t *attr" "int *shared"
+.Fc
 .Ft int
 .Fn pthread_mutexattr_setrobust "pthread_mutexattr_t *attr" "int robust"
 .Ft int
@@ -64,7 +85,9 @@
 .Ft int
 .Fn pthread_mutexattr_settype "pthread_mutexattr_t *attr" "int type"
 .Ft int
-.Fn pthread_mutexattr_gettype "const pthread_mutexattr_t *restrict attr" "int *restrict type"
+.Fo pthread_mutexattr_gettype
+.Fa "const pthread_mutexattr_t *restrict attr" "int *restrict type"
+.Fc
 .Sh DESCRIPTION
 Mutex attributes are used to specify parameters to
 .Fn pthread_mutex_init .
@@ -84,8 +107,89 @@
 .Fa attr .
 .Pp
 The
-.Fn pthread_mutexattr_set*
-functions set the attribute that corresponds to each function name.
+.Fn pthread_mutexattr_setprioceiling
+function set the priority ceiling for the mutex, used
+by threads executed under
+.Dv PTHREAD_PRIO_PROTECT
+protocol.
+.Pp
+The
+.Fn pthread_mutexattr_setprotocol
+function specifies the protocol to be followed in utilizing mutexes.
+The
+.Fa protocol
+argument can take one of the following values:
+.Bl -tag -width PTHREAD_PRIO_PROTECT
+.It PTHREAD_PRIO_NONE
+Priority and scheduling of the thread owning this mutex is not
+affected by its mutex ownership.
+.It PTHREAD_PRIO_INHERIT
+Request priority-inheritance protocol, where the thread owning the mutex
+is executed at highest priority among priorities of all threads waiting
+on any mutex owned by this thread.
+.It PTHREAD_PRIO_PROTECT
+Request priority-inheritance protocol, where the thread owning the mutex
+is executed at highest priority among priorities or priority ceilings of
+all threads waiting on any mutex owned by this thread.
+.El
+.Pp
+The
+.Fn pthread_mutexattr_setrobust
+function specifies robust attribute of the mutex.
+Possible values for the
+.Fa robust
+argument are
+.Bl -tag -width PTHREAD_MUTEX_STALLED
+.It PTHREAD_MUTEX_STALLED
+No special actions are taken if the thread owning the mutex is terminated
+while holding the mutex lock.
+This can lead to deadlocks if no other thread can unlock the mutex.
+This is the default value.
+.It PTHREAD_MUTEX_ROBUST
+If the process containing the owning thread of a robust mutex, or owning
+thread, terminates while holding the mutex lock, the next thread that
+acquires the mutex is notified about the termination
+by the return value
+.Ev EOWNERDEAD
+from the locking function.
+Then, either
+.Xr pthread_mutex_consistent 3
+can be used to repair the mutex lock state, or
+.Xr pthread_mutex_unlock 3
+can unlock the mutex lock but also put it unreparaible state,
+where all further attempts to acquire it result in the
+.Ev ENOTRECOVERABLE
+error.
+.El
+.Pp
+The
+.Fn pthread_mutexattr_settype
+sets the type of the mutex.
+Type affects the behavior of calls which lock and unlock the mutex.
+The possible values for the
+.Fa type
+argument are
+.Bl -tag -width PTHREAD_MUTEX_ERRORCHECK
+.It PTHREAD_MUTEX_NORMAL
+Both recursive locking and unlock when the lock is not owned by the current
+thread cause an error returned from the corresponding functions.
+This matches
+.Dv PTHREAD_MUTEX_ERRORCHECK
+but somewhat contradicts to the behavior mandated by POSIX.
+.It PTHREAD_MUTEX_ERRORCHECK
+Both recursive locking and unlock when the lock is not owned by the current
+thread cause an error returned from the corresponding functions.
+.It PTHREAD_MUTEX_RECURSIVE
+Recursive locking is allowed.
+Attempt to unlock when current thread is not an owner of the lock results
+in an error returned.
+.It PTHREAD_MUTEX_DEFAULT
+The
+.Fx
+implementation maps this type to
+.Dv PTHREAD_MUTEX_ERRORCHECK
+type.
+.El
 .Pp
 The
 .Fn pthread_mutexattr_get*
@@ -153,6 +257,26 @@
 .El
 .Pp
 The
+.Fn pthread_mutexattr_setpshared
+function will fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+Invalid value for
+.Fa attr ,
+or invalid value for
+.Fa shared .
+.El
+.Pp
+The
+.Fn pthread_mutexattr_getpshared
+function will fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+Invalid value for
+.Fa attr .
+.El
+.Pp
+The
 .Fn pthread_mutexattr_settype
 function will fail if:
 .Bl -tag -width Er
@@ -206,6 +330,8 @@
 .Fn pthread_mutexattr_getprioceiling ,
 .Fn pthread_mutexattr_setprotocol ,
 .Fn pthread_mutexattr_getprotocol ,
+.Fn pthread_mutexattr_setpshared ,
+.Fn pthread_mutexattr_getpshared ,
 .Fn pthread_mutexattr_settype ,
 and
 .Fn pthread_mutexattr_gettype
diff --git a/share/man/man3/pthread_rwlockattr_getpshared.3 b/share/man/man3/pthread_rwlockattr_getpshared.3
--- a/share/man/man3/pthread_rwlockattr_getpshared.3
+++ b/share/man/man3/pthread_rwlockattr_getpshared.3
@@ -35,7 +35,9 @@
 .Sh SYNOPSIS
 .In pthread.h
 .Ft int
-.Fn pthread_rwlockattr_getpshared "const pthread_rwlockattr_t *restrict attr" "int *restrict pshared"
+.Fo pthread_rwlockattr_getpshared
+.Fa "const pthread_rwlockattr_t *restrict attr" "int *restrict pshared"
+.Fc
 .Sh DESCRIPTION
 The
 .Fn pthread_rwlockattr_getpshared