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mutex_init(3c)
Standard C Library Functions mutex_init(3C)
NAME
mutex_init, mutex_lock, mutex_trylock, mutex_unlock, mutex_consistent,
mutex_destroy - mutual exclusion locks
SYNOPSIS
#include <synch.h>
int mutex_init(mutex_t *mp, int type, void *arg);
int mutex_lock(mutex_t *mp);
int mutex_trylock(mutex_t *mp);
int mutex_unlock(mutex_t *mp);
int mutex_consistent(mutex_t *mp);
int mutex_destroy(mutex_t *mp);
DESCRIPTION
Mutual exclusion locks (mutexes) prevent multiple threads from simulta‐
neously executing critical sections of code that access shared data
(that is, mutexes are used to serialize the execution of threads). All
mutexes must be global. A successful call for a mutex lock by way of
mutex_lock() will cause another thread that is also trying to lock the
same mutex to block until the owner thread unlocks it by way of
mutex_unlock(). Threads within the same process or within other pro‐
cesses can share mutexes.
Mutexes can synchronize threads within the same process or in other
processes. Mutexes can be used to synchronize threads between processes
if the mutexes are allocated in writable memory and shared among the
cooperating processes (see mmap(2)), and have been initialized for this
task.
Initialize
Mutexes are either intra-process or inter-process, depending upon the
argument passed implicitly or explicitly to the initialization of that
mutex. A statically allocated mutex does not need to be explicitly ini‐
tialized; by default, a statically allocated mutex is initialized with
all zeros and its scope is set to be within the calling process.
For inter-process synchronization, a mutex needs to be allocated in
memory shared between these processes. Since the memory for such a
mutex must be allocated dynamically, the mutex needs to be explicitly
initialized using mutex_init().
The mutex_init() function initializes the mutex referenced by mp with
the type specified by type. Upon successful initialization the state of
the mutex becomes initialized and unlocked. Only the attribute type
LOCK_PRIO_PROTECT uses arg. The type argument must be one of the fol‐
lowing:
USYNC_THREAD
The mutex can synchronize threads only in this process.
USYNC_PROCESS
The mutex can synchronize threads in this process and other pro‐
cesses. The object initialized with this attribute must be allo‐
cated in memory shared between processes, either in System V shared
memory (see shmop(2)) or in memory mapped to a file (see mmap(2)).
If the object is not allocated in such shared memory, it will not
be shared between processes.
The type argument can be augmented by the bitwise-inclusive-OR of zero
or more of the following flags:
LOCK_ROBUST
The mutex can synchronize threads robustly. At the time of thread
or process death, either by calling thr_exit() or exit() or due to
process abnormal termination, the lock is unlocked if is held by
the thread or process. The next owner of the mutex will acquire it
with an error return of EOWNERDEAD. The application must always
check the return value from mutex_lock() for a mutex of this type.
The new owner of this mutex should then attempt to make the state
protected by the mutex consistent, since this state could have been
left inconsistent when the last owner died. If the new owner is
able to make the state consistent, it should call mutex_consis‐
tent() to restore the state of the mutex and then unlock the mutex.
All subsequent calls to mutex_lock()will then behave normally. Only
the new owner can make the mutex consistent. If for any reason the
new owner is not able to make the state consistent, it should not
call mutex_consistent() but should simply unlock the mutex. All
waiting processes will be awakened and all subsequent calls to
mutex_lock() will fail in acquiring the mutex with an error value
of ENOTRECOVERABLE. If the thread or process that acquired the lock
with EOWNERDEAD terminates without unlocking the mutex, the next
owner will acquire the lock with an error value of EOWNERDEAD.
The memory for the object to be initialized with this attribute
must be zeroed before initialization. Any thread or process inter‐
ested in the robust lock can call mutex_init() to potentially ini‐
tialize it, provided that all such callers of mutex_init() specify
the same set of attribute flags. In this situation, if mutex_init()
is called on a previously initialized robust mutex, mutex_init()
will not reinitialize the mutex and will return the error value
EBUSY.
LOCK_RECURSIVE
A thread attempting to relock this mutex without first unlocking it
will succeed in locking the mutex. The mutex must be unlocked as
many times as it is locked.
LOCK_ERRORCHECK
Unless LOCK_RECURSIVE is also set, a thread attempting to relock
this mutex without first unlocking it will return with an error
rather than deadlocking itself. A thread attempting to unlock this
mutex without first owning it will return with an error.
LOCK_PRIO_INHERIT
When a thread is blocking higher priority threads because of owning
one or more mutexes with the LOCK_PRIO_INHERIT attribute, it exe‐
cutes at the higher of its priority or the priority of the highest
priority thread waiting on any of the mutexes owned by this thread
and initialized with this attribute.
LOCK_PRIO_PROTECT
When a thread owns one or more mutexes initialized with the
LOCK_PRIO_PROTECT attribute, it executes at the higher of its pri‐
ority or the highest of the priority ceilings of all the mutexes
owned by this thread and initialized with this attribute, regard‐
less of whether other threads are blocked on any of these mutexes.
When this attribute is specified, arg must point to an int contain‐
ing the priority ceiling.
See pthread_mutexattr_getrobust(3C) for more information about robust
mutexes. The LOCK_ROBUST attribute is the same as the POSIX
PTHREAD_MUTEX_ROBUST attribute.
See pthread_mutexattr_settype(3C) for more information on recursive and
error checking mutex types. The combination (LOCK_RECURSIVE |
LOCK_ERRORCHECK) is the same as the POSIX PTHREAD_MUTEX_RECURSIVE type.
By itself, LOCK_ERRORCHECK is the same as the POSIX
PTHREAD_MUTEX_ERRORCHECK type.
The LOCK_PRIO_INHERIT attribute is the same as the POSIX
PTHREAD_PRIO_INHERIT attribute. The LOCK_PRIO_PROTECT attribute is the
same as the POSIX PTHREAD_PRIO_PROTECT attribute. See pthread_mutex‐
attr_getprotocol(3C), pthread_mutexattr_getprioceiling(3C), and
pthread_mutex_getprioceiling(3C) for a full discussion. The
LOCK_PRIO_INHERIT and LOCK_PRIO_PROTECT attributes are mutually exclu‐
sive. Specifying both of these attributes causes mutex_init() to fail
with EINVAL.
Initializing mutexes can also be accomplished by allocating in zeroed
memory (default), in which case a type of USYNC_THREAD is assumed. In
general, the following rules apply to mutex initialization:
o The same mutex must not be simultaneously initialized by
multiple threads.
o A mutex lock must not be reinitialized while in use by other
threads.
These rules do not apply to LOCK_ROBUST mutexes. See the description
for LOCK_ROBUST above. If default mutex attributes are used, the macro
DEFAULTMUTEX can be used to initialize mutexes that are statically
allocated.
Default mutex initialization (intra-process):
mutex_t mp;
mutex_init(&mp, USYNC_THREAD, NULL);
or
mutex_t mp = DEFAULTMUTEX;
Customized mutex initialization (inter-process):
mutex_init(&mp, USYNC_PROCESS, NULL);
Customized mutex initialization (inter-process robust):
mutex_init(&mp, USYNC_PROCESS | LOCK_ROBUST, NULL);
Statically allocated mutexes can also be initialized with macros speci‐
fying LOCK_RECURSIVE and/or LOCK_ERRORCHECK:
mutex_t mp = RECURSIVEMUTEX;
Same as (USYNC_THREAD | LOCK_RECURSIVE)
mutex_t mp = ERRORCHECKMUTEX;
Same as (USYNC_THREAD | LOCK_ERRORCHECK)
mutex_t mp = RECURSIVE_ERRORCHECKMUTEX;
Same as (USYNC_THREAD | LOCK_RECURSIVE | LOCK_ERRORCHECK)
Lock and Unlock
A critical section of code is enclosed by a the call to lock the mutex
and the call to unlock the mutex to protect it from simultaneous access
by multiple threads. Only one thread at a time may possess mutually
exclusive access to the critical section of code that is enclosed by
the mutex-locking call and the mutex-unlocking call, whether the
mutex's scope is intra-process or inter-process. A thread calling to
lock the mutex either gets exclusive access to the code starting from
the successful locking until its call to unlock the mutex, or it waits
until the mutex is unlocked by the thread that locked it.
Mutexes have ownership, unlike semaphores. Although any thread, within
the scope of a mutex, can get an unlocked mutex and lock access to the
same critical section of code, only the thread that locked a mutex
should unlock it.
If a thread waiting for a mutex receives a signal, upon return from the
signal handler, the thread resumes waiting for the mutex as if there
was no interrupt. A mutex protects code, not data; therefore, strongly
bind a mutex with the data by putting both within the same structure,
or at least within the same procedure.
A call to mutex_lock() locks the mutex object referenced by mp. If the
mutex is already locked, the calling thread blocks until the mutex is
freed; this will return with the mutex object referenced by mp in the
locked state with the calling thread as its owner. If the current owner
of a mutex tries to relock the mutex, it will result in deadlock.
The mutex_trylock() function is the same as mutex_lock(), respectively,
except that if the mutex object referenced by mp is locked (by any
thread, including the current thread), the call returns immediately
with an error.
The mutex_unlock() function are called by the owner of the mutex object
referenced by mp to release it. The mutex must be locked and the call‐
ing thread must be the one that last locked the mutex (the owner). If
there are threads blocked on the mutex object referenced by mp when
mutex_unlock() is called, the mp is freed, and the scheduling policy
will determine which thread gets the mutex. If the calling thread is
not the owner of the lock, no error status is returned, and the behav‐
ior of the program is undefined.
Destroy
The mutex_destroy() function destroys the mutex object referenced by
mp. The mutex object becomes uninitialized. The space used by the
destroyed mutex variable is not freed. It needs to be explicitly
reclaimed.
RETURN VALUES
If successful, these functions return 0. Otherwise, an error number is
returned.
ERRORS
The mutex_init() function will fail if:
EINVAL The value specified by type is invalid, or the
LOCK_PRIO_INHERIT and LOCK_PRIO_PROTECT attributes are both
specified.
The mutex_init() function will fail for LOCK_ROBUST type mutex if:
EBUSY The mutex pointed to by mp was previously initialized and has
not yet been destroyed.
EINVAL The mutex pointed to by mp was previously initialized with a
different set of attribute flags.
The mutex_trylock() function will fail if:
EBUSY The mutex pointed to by mp is already locked.
The mutex_lock() and mutex_trylock() functions will fail for a
LOCK_RECURSIVE mutex if:
EAGAIN The mutex could not be acquired because the maximum number of
recursive locks for the mutex has been reached.
The mutex_lock() function will fail for a LOCK_ERRORCHECK and non-
LOCK_RECURSIVE mutex if:
EDEADLK The caller already owns the mutex.
The mutex_lock() function may fail for a non-LOCK_ERRORCHECK and non-
LOCK_RECURSIVE mutex if:
EDEADLK The caller already owns the mutex.
The mutex_unlock() function will fail for a LOCK_ERRORCHECK mutex if:
EPERM The caller does not own the mutex.
The mutex_lock() or mutex_trylock() functions will fail for LOCK_ROBUST
type mutex if:
EOWNERDEAD The last owner of this mutex died while holding the
mutex. This mutex is now owned by the caller. The
caller must now attempt to make the state protected
by the mutex consistent. If it is able to clean up
the state, then it should restore the state of the
mutex by calling mutex_consistent() and unlock the
mutex. Subsequent calls to mutex_lock() will behave
normally, as before. If the caller is not able to
clean up the state, mutex_consistent() should not be
called but the mutex should be unlocked. Subsequent
calls to mutex_lock() will fail to acquire the
mutex, returning with the error value ENOTRECOVER‐
ABLE. If the owner who acquired the lock with EOWN‐
ERDEAD dies, the next owner will acquire the lock
with EOWNERDEAD.
ENOTRECOVERABLE The mutex trying to be acquired was protecting the
state that has been left unrecoverable when the
mutex's last owner could not make the state pro‐
tected by the mutex consistent. The mutex has not
been acquired. This condition occurs when the lock
was previously acquired with EOWNERDEAD and the
owner was not able to clean up the state and
unlocked the mutex without calling mutex_consis‐
tent().
The mutex_consistent() function will fail if:
EINVAL The caller does not own the mutex or the mutex is not a
LOCK_ROBUST mutex having an inconsistent state (EOWNERDEAD).
EXAMPLES
Single Gate
The following example uses one global mutex as a gate-keeper to permit
each thread exclusive sequential access to the code within the user-
defined function "change_global_data." This type of synchronization
will protect the state of shared data, but it also prohibits parallel‐
ism.
/* cc thisfile.c */
#include <stdio.h>
#include <thread.h>
#define NUM_THREADS 12
void *change_global_data(void *); /* for thr_create() */
main(int argc,char * argv[]) {
int i=0;
for (i=0; i< NUM_THREADS; i++) {
thr_create(NULL, 0, change_global_data, NULL, 0, NULL);
}
while ((thr_join(NULL, NULL, NULL) == 0));
}
void * change_global_data(void *null){
static mutex_t Global_mutex;
static int Global_data = 0;
mutex_lock(&Global_mutex);
Global_data++;
sleep(1);
printf("%d is global data\n",Global_data);
mutex_unlock(&Global_mutex);
return NULL;
}
Multiple Instruction Single Data
The previous example, the mutex, the code it owns, and the data it pro‐
tects was enclosed in one function. The next example uses C++ features
to accommodate many functions that use just one mutex to protect one
data:
/* CC thisfile.c use C++ to compile*/
#include <stdlib.h>
#include <stdio.h>
#include <thread.h>
#include <errno.h>
#include <iostream.h>
#define NUM_THREADS 16
void *change_global_data(void *); /* for thr_create() */
class Mutected {
private:
static mutex_t Global_mutex;
static int Global_data;
public:
static int add_to_global_data(void);
static int subtract_from_global_data(void);
};
int Mutected::Global_data = 0;
mutex_t Mutected::Global_mutex;
int Mutected::add_to_global_data() {
mutex_lock(&Global_mutex);
Global_data++;
mutex_unlock(&Global_mutex);
return Global_data;
}
int Mutected::subtract_from_global_data() {
mutex_lock(&Global_mutex);
Global_data--;
mutex_unlock(&Global_mutex);
return Global_data;
}
void
main(int argc,char * argv[]) {
int i=0;
for (i=0;i< NUM_THREADS;i++) {
thr_create(NULL,0,change_global_data,NULL,0,NULL);
}
while ((thr_join(NULL,NULL,NULL) == 0));
}
void * change_global_data(void *) {
static int switcher = 0;
if ((switcher++ % 3) == 0) /* one-in-three threads subtracts */
cout << Mutected::subtract_from_global_data() << endl;
else
cout << Mutected::add_to_global_data() << endl;
return NULL;
}
Interprocess Locking
A mutex can protect data that is shared among processes. The mutex
would need to be initialized as USYNC_PROCESS. One process initializes
the process-shared mutex and writes it to a file to be mapped into mem‐
ory by all cooperating processes (see mmap(2)). Afterward, other inde‐
pendent processes can run the same program (whether concurrently or
not) and share mutex-protected data.
/* cc thisfile.c */
/* To execute, run the command line "a.out 0 &; a.out 1" */
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <thread.h>
#define INTERPROCESS_FILE "ipc-sharedfile"
#define NUM_ADDTHREADS 12
#define NUM_SUBTRACTTHREADS 10
#define INCREMENT '0'
#define DECREMENT '1'
typedef struct {
mutex_t Interprocess_mutex;
int Interprocess_data;
} buffer_t;
buffer_t *buffer;
void *add_interprocess_data(), *subtract_interprocess_data();
void create_shared_memory(), test_argv();
int zeroed[sizeof(buffer_t)];
int ipc_fd, i=0;
void
main(int argc,char * argv[]){
test_argv(argv[1]);
switch (*argv[1]) {
case INCREMENT:
/* Initializes the process-shared mutex */
/* Should be run prior to running a DECREMENT process */
create_shared_memory();
ipc_fd = open(INTERPROCESS_FILE, O_RDWR);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
buffer->Interprocess_data = 0;
mutex_init(&buffer->Interprocess_mutex, USYNC_PROCESS,0);
for (i=0; i< NUM_ADDTHREADS; i++)
thr_create(NULL, 0, add_interprocess_data, argv[1],
0, NULL);
break;
case DECREMENT:
/* Should be run after the INCREMENT process has run. */
while(ipc_fd = open(INTERPROCESS_FILE, O_RDWR)) == -1)
sleep(1);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
for (i=0; i< NUM_SUBTRACTTHREADS; i++)
thr_create(NULL, 0, subtract_interprocess_data, argv[1],
0, NULL);
break;
} /* end switch */
while ((thr_join(NULL,NULL,NULL) == 0));
} /* end main */
void *add_interprocess_data(char argv_1[]){
mutex_lock(&buffer->Interprocess_mutex);
buffer->Interprocess_data++;
sleep(2);
printf("%d is add-interprocess data, and %c is argv1\n",
buffer->Interprocess_data, argv_1[0]);
mutex_unlock(&buffer->Interprocess_mutex);
return NULL;
}
void *subtract_interprocess_data(char argv_1[]) {
mutex_lock(&buffer->Interprocess_mutex);
buffer->Interprocess_data--;
sleep(2);
printf("%d is subtract-interprocess data, and %c is argv1\n",
buffer->Interprocess_data, argv_1[0]);
mutex_unlock(&buffer->Interprocess_mutex);
return NULL;
}
void create_shared_memory(){
int i;
ipc_fd = creat(INTERPROCESS_FILE, O_CREAT | O_RDWR );
for (i=0; i<sizeof(buffer_t); i++){
zeroed[i] = 0;
write(ipc_fd, &zeroed[i],2);
}
close(ipc_fd);
chmod(INTERPROCESS_FILE, S_IRWXU | S_IRWXG | S_IRWXO);
}
void test_argv(char argv1[]) {
if (argv1 == NULL) {
printf("use 0 as arg1 for initial process\n \
or use 1 as arg1 for the second process\n");
exit(NULL);
}
}
Solaris Interprocess Robust Locking
A mutex can protect data that is shared among processes robustly. The
mutex would need to be initialized as USYNC_PROCESS | LOCK_ROBUST. One
process initializes the robust process-shared mutex and writes it to a
file to be mapped into memory by all cooperating processes (see
mmap(2)). Afterward, other independent processes can run the same pro‐
gram (whether concurrently or not) and share mutex-protected data.
The following example shows how to use a USYNC_PROCESS | LOCK_ROBUST
type mutex.
/* cc thisfile.c */
/* To execute, run the command line "a.out & a.out 1" */
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <thread.h>
#define INTERPROCESS_FILE "ipc-sharedfile"
typedef struct {
mutex_t Interprocess_mutex;
int Interprocess_data;
} buffer_t;
buffer_t *buffer;
int make_date_consistent();
void create_shared_memory();
int zeroed[sizeof(buffer_t)];
int ipc_fd, i=0;
main(int argc,char * argv[]) {
int rc;
if (argc > 1) {
while((ipc_fd = open(INTERPROCESS_FILE, O_RDWR)) == -1)
sleep(1);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
mutex_init(&buffer->Interprocess_mutex,
USYNC_PROCESS | LOCK_ROBUST,0);
} else {
create_shared_memory();
ipc_fd = open(INTERPROCESS_FILE, O_RDWR);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
buffer->Interprocess_data = 0;
mutex_init(&buffer->Interprocess_mutex,
USYNC_PROCESS | LOCK_ROBUST,0);
}
for(;;) {
rc = mutex_lock(&buffer->Interprocess_mutex);
switch (rc) {
case EOWNERDEAD:
/*
* The lock is acquired.
* The last owner died holding the lock.
* Try to make the state associated with
* the mutex consistent.
* If successful, make the robust lock consistent.
*/
if (make_data_consistent())
mutex_consistent(&buffer->Interprocess_mutex);
mutex_unlock(&buffer->Interprocess_mutex);
break;
case ENOTRECOVERABLE:
/*
* The lock is not acquired.
* The last owner got the mutex with EOWNERDEAD
* and failed to make the data consistent.
* There is no way to recover, so just exit.
*/
exit(1);
case 0:
/*
* There is no error - data is consistent.
* Do something with data.
*/
mutex_unlock(&buffer->Interprocess_mutex);
break;
}
}
} /* end main */
void create_shared_memory() {
int i;
ipc_fd = creat(INTERPROCESS_FILE, O_CREAT | O_RDWR );
for (i=0; i<sizeof(buffer_t); i++) {
zeroed[i] = 0;
write(ipc_fd, &zeroed[i],2);
}
close(ipc_fd);
chmod(INTERPROCESS_FILE, S_IRWXU | S_IRWXG | S_IRWXO);
}
/* return 1 if able to make data consistent, otherwise 0. */
int make_data_consistent () {
buffer->Interprocess_data = 0;
return (1);
}
Dynamically Allocated Mutexes
The following example allocates and frees memory in which a mutex is
embedded.
struct record {
int field1;
int field2;
mutex_t m;
} *r;
r = malloc(sizeof(struct record));
mutex_init(&r->m, USYNC_THREAD, NULL);
/*
* The fields in this record are accessed concurrently
* by acquiring the embedded lock.
*/
The thread execution in this example is as follows:
Thread 1 executes: Thread 2 executes:
... ...
mutex_lock(&r->m); mutex_lock(&r->m);
r->field1++; localvar = r->field1;
mutex_unlock(&r->m); mutex_unlock(&r->m);
... ...
Later, when a thread decides to free the memory pointed to by r, the
thread should call mutex_destroy() on the mutexes in this memory.
In the following example, the main thread can do a thr_join() on both
of the above threads. If there are no other threads using the memory in
r, the main thread can now safely free r:
for (i = 0; i < 2; i++)
thr_join(0, 0, 0);
mutex_destroy(&r->m); /* first destroy mutex */
free(r); /* then free memory */
If the mutex is not destroyed, the program could have memory leaks.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe
SEE ALSO
mmap(2), shmop(2), pthread_mutex_getprioceiling(3C),
pthread_mutex_init(3C), pthread_mutexattr_getprioceiling(3C),
pthread_mutexattr_getprotocol(3C), pthread_mutexattr_getrobust(3C),
pthread_mutexattr_gettype(3C), attributes(7), mutex(7), standards(7)
NOTES
Previous releases of Solaris provided the USYNC_PROCESS_ROBUST mutex
type. This type is now deprecated but is still supported for source and
binary compatibility. When passed to mutex_init(), it is transformed
into (USYNC_PROCESS | LOCK_ROBUST). The former method for restoring a
USYNC_PROCESS_ROBUST mutex to a consistent state was to reinitialize it
by calling mutex_init(). This method is still supported for source and
binary compatibility, but the proper method is to call mutex_consis‐
tent().
The USYNC_PROCESS_ROBUST type permitted an alternate error value,
ELOCKUNMAPPED, to be returned by mutex_lock() if the process containing
a locked robust mutex unmapped the memory containing the mutex or per‐
formed one of the exec(2) functions. The ELOCKUNMAPPED error value
implies all of the consequences of the EOWNERDEAD error value and as
such is just a synonym for EOWNERDEAD. For full source and binary com‐
patibility, the ELOCKUNMAPPED error value is still returned from
mutex_lock() in these circumstances, but only if the mutex was initial‐
ized with the USYNC_PROCESS_ROBUST type. Otherwise, EOWNERDEAD is
returned in these circumstances.
The mutex_lock(), mutex_unlock(), and mutex_trylock() functions do not
validate the mutex type. An uninitialized mutex or a mutex with an
invalid type does not return EINVAL. Interfaces for mutexes with an
invalid type have unspecified behavior.
Uninitialized mutexes that are allocated locally could contain junk
data. Such mutexes need to be initialized using mutex_init().
By default, if multiple threads are waiting for a mutex, the order of
acquisition is undefined.
Oracle Solaris 11.4 11 May 2021 mutex_init(3C)