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STRUCTURES

The operations, performed by the _umtx_op() syscall, operate on userspace objects which are described by the following structures. Reserved fields and paddings are omitted. All objects require ABI-mandated alignment, but this is not currently enforced consistently on all architectures.

The following flags are defined for flag fields of all structures:

USYNC_PROCESS_SHARED

Allow selection of the process-shared sleep queue for the thread sleep container, when the lock ownership cannot be granted immediately, and the operation must sleep. The process-shared or process-private sleep queue is selected based on the attributes of the memory mapping which contains the first byte of the structure, see mmap(2). Otherwise, if the flag is not specified, the process-private sleep queue is selected regardless of the memory mapping attributes, as an optimization.

See the SLEEP QUEUES subsection below for more details on sleep queues.

Mutex

struct umutex {
	volatile lwpid_t m_owner;
	uint32_t         m_flags;
	uint32_t         m_ceilings[2];
	uintptr_t        m_rb_lnk;
};

The m_owner field is the actual lock. It contains either the thread identifier of the lock owner in the locked state, or zero when the lock is unowned. The highest bit set indicates that there is contention on the lock. The constants are defined for special values:

UMUTEX_UNOWNED

Zero, the value stored in the unowned lock.

UMUTEX_CONTESTED

The contenion indicator.

UMUTEX_RB_OWNERDEAD

A thread owning the robust mutex terminated. The mutex is in unlocked state.

UMUTEX_RB_NOTRECOV

The robust mutex is in a non-recoverable state. It cannot be locked until reinitialized.

The m_flags field may contain the following umutex-specific flags, in addition to the common flags:

UMUTEX_PRIO_INHERIT

Mutex implements Priority Inheritance protocol.

UMUTEX_PRIO_PROTECT

Mutex implements Priority Protection protocol.

UMUTEX_ROBUST

Mutex is robust, as described in the ROBUST UMUTEXES section below.

UMUTEX_NONCONSISTENT

Robust mutex is in a transient non-consistent state. Not used by kernel.

In the manual page, mutexes not having UMUTEX_PRIO_INHERIT and UMUTEX_PRIO_PROTECT flags set, are called normal mutexes. Each type of mutex (normal, priority-inherited, and priority-protected) has a separate sleep queue associated with the given key.

For priority protected mutexes, the m_ceilings array contains priority ceiling values. The m_ceilings[0] is the ceiling value for the mutex, as specified by IEEE Std 1003.1-2008 (“POSIX.1”) for the Priority Protected mutex protocol. The m_ceilings[1] is used only for the unlock of a priority protected mutex, when unlock is done in an order other than the reversed lock order. In this case, m_ceilings[1] must contain the ceiling value for the last locked priority protected mutex, for proper priority reassignment. If, instead, the unlocking mutex was the last priority propagated mutex locked by the thread, m_ceilings[1] should contain -1. This is required because kernel does not maintain the ordered lock list.

Condition variable

struct ucond {
	volatile uint32_t c_has_waiters;
	uint32_t          c_flags;
	uint32_t          c_clockid;
};

A non-zero c_has_waiters value indicates that there are in-kernel waiters for the condition, executing the UMTX_OP_CV_WAIT request.

The c_flags field contains flags. Only the common flags (USYNC_PROCESS_SHARED) are defined for ucond.

The c_clockid member provides the clock identifier to use for timeout, when the UMTX_OP_CV_WAIT request has both the CVWAIT_CLOCKID flag and the timeout specified. Valid clock identifiers are a subset of those for clock_gettime(2):

• CLOCK_MONOTONIC
• CLOCK_MONOTONIC_FAST
• CLOCK_MONOTONIC_PRECISE
• CLOCK_PROF
• CLOCK_REALTIME
• CLOCK_REALTIME_FAST
• CLOCK_REALTIME_PRECISE
• CLOCK_SECOND
• CLOCK_UPTIME
• CLOCK_UPTIME_FAST
• CLOCK_UPTIME_PRECISE
• CLOCK_VIRTUAL

Reader/writer lock

struct urwlock {
	volatile int32_t rw_state;
	uint32_t         rw_flags;
	uint32_t         rw_blocked_readers;
	uint32_t         rw_blocked_writers;
};

The rw_state field is the actual lock. It contains both the flags and counter of the read locks which were granted. Names of the rw_state bits are following:

URWLOCK_WRITE_OWNER

Write lock was granted.

URWLOCK_WRITE_WAITERS

There are write lock waiters.

URWLOCK_READ_WAITERS

There are read lock waiters.

URWLOCK_READER_COUNT(c)

Returns the count of currently granted read locks.

At any given time there may be only one thread to which the writer lock is granted on the struct rwlock, and no threads are granted read lock. Or, at the given time, up to URWLOCK_MAX_READERS threads may be granted the read lock simultaneously, but write lock is not granted to any thread.

The following flags for the rw_flags member of struct urwlock are defined, in addition to the common flags:

URWLOCK_PREFER_READER

If specified, immediately grant read lock requests when urwlock is already read-locked, even in presence of unsatisfied write lock requests. By default, if there is a write lock waiter, further read requests are not granted, to prevent unfair write lock waiter starvation.

The rw_blocked_readers and rw_blocked_writers members contain the count of threads which are sleeping in kernel, waiting for the associated request type to be granted. The fields are used by kernel to update the URWLOCK_READ_WAITERS and URWLOCK_WRITE_WAITERS flags of the rw_state lock after requesting thread was woken up.

Semaphore

struct _usem2 {
	volatile uint32_t _count;
	uint32_t          _flags;
};

The _count word represents a counting semaphore. A non-zero value indicates an unlocked (posted) semaphore, while zero represents the locked state. The maximal supported semaphore count is USEM_MAX_COUNT.

The _count word, besides the counter of posts (unlocks), also contains the USEM_HAS_WAITERS bit, which indicates that locked semaphore has waiting threads.

The USEM_COUNT() macro, applied to the _count word, returns the current semaphore counter, which is the number of posts issued on the semaphore.

The following bits for the _flags member of struct _usem2 are defined, in addition to the common flags:

USEM_NAMED

Flag is ignored by kernel.

Timeout parameter

struct _umtx_time {
	struct timespec _timeout;
	uint32_t        _flags;
	uint32_t        _clockid;
};

Several _umtx_op() operations allow the blocking time to be limited, failing the request if it cannot be satisfied in the specified time period. The timeout is specified by passing either the address of struct timespec, or its extended variant, struct _umtx_time, as the uaddr2 argument of _umtx_op(). They are distinguished by the uaddr value, which must be equal to the size of the structure pointed to by uaddr2, casted to uintptr_t.

The _timeout member specifies the time when the timeout should occur. Legal values for clock identifier _clockid are shared with the clock_id argument to the clock_gettime(2) function, and use the same underlying clocks. The specified clock is used to obtain the current time value. Interval counting is always performed by the monotonic wall clock.

The _flags argument allows the following flags to further define the timeout behaviour:

UMTX_ABSTIME

The _timeout value is the absolute time. The thread will be unblocked and the request failed when specified clock value is equal or exceeds the _timeout.

If the flag is absent, the timeout value is relative, that is the amount of time, measured by the monotonic wall clock from the moment of the request start.

SLEEP QUEUES

When a locking request cannot be immediately satisfied, the thread is typically put to sleep, which is a non-runnable state terminated by the wake operation. Lock operations include a try variant which returns an error rather than sleeping if the lock cannot be obtained. Also, _umtx_op() provides requests which explicitly put the thread to sleep.

Wakes need to know which threads to make runnable, so sleeping threads are grouped into containers called sleep queues. A sleep queue is identified by a key, which for _umtx_op() is defined as the physical address of some variable. Note that the physical address is used, which means that same variable mapped multiple times will give one key value. This mechanism enables the construction of process-shared locks.

A related attribute of the key is shareability. Some requests always interpret keys as private for the current process, creating sleep queues with the scope of the current process even if the memory is shared. Others either select the shareability automatically from the mapping attributes, or take additional input as the USYNC_PROCESS_SHARED common flag. This is done as optimization, allowing the lock scope to be limited regardless of the kind of backing memory.

Only the address of the start byte of the variable specified as key is important for determining corresponding sleep queue. The size of the variable does not matter, so, for example, sleep on the same address interpeted as uint32_t and long on a little-endian 64-bit platform would collide.

The last attribute of the key is the object type. The sleep queue to which a sleeping thread is assigned is an individual one for simple wait requests, mutexes, rwlocks, condvars and other primitives, even when the physical address of the key is same.

When waking up a limited number of threads from a given sleep queue, the highest priority threads that have been blocked for the longest on the queue are selected.

ROBUST UMUTEXES

The robust umutexes are provided as a substrate for a userspace library to implement POSIX robust mutexes. A robust umutex must have the UMUTEX_ROBUST flag set.

On thread termination, the kernel walks two lists of mutexes. The two lists head addresses must be provided by a prior call to UMTX_OP_ROBUST_LISTS request. The lists are singly-linked. The link to next element is provided by the m_rb_lnk member of the struct umutex.

Robust list processing is aborted if the kernel finds a mutex with any of the following conditions:

• the UMUTEX_ROBUST flag is not set
• not owned by the current thread, except when the mutex is pointed to by the robust_inactive member of the struct umtx_robust_lists_params, registered for the current thread
• the combination of mutex flags is invalid
• read of the umutex memory faults
• the list length limit described in libthr(3) is reached.

Every mutex in both lists is unlocked as if the UMTX_OP_MUTEX_UNLOCK request is performed on it, but instead of the UMUTEX_UNOWNED value, the m_owner field is written with the UMUTEX_RB_OWNERDEAD value. When a mutex in the UMUTEX_RB_OWNERDEAD state is locked by kernel due to the UMTX_OP_MUTEX_TRYLOCK and UMTX_OP_MUTEX_LOCK requests, the lock is granted and EOWNERDEAD error is returned.

Also, the kernel handles the UMUTEX_RB_NOTRECOV value of the m_owner field specially, always returning the ENOTRECOVERABLE error for lock attempts, without granting the lock.

OPERATIONS

The following operations, requested by the op argument to the function, are implemented:

UMTX_OP_WAIT

Wait. The arguments for the request are:

obj

Pointer to a variable of type long.

val

Current value of the *obj.

The current value of the variable pointed to by the obj argument is compared with the val. If they are equal, the requesting thread is put to interruptible sleep until woken up or the optionally specified timeout expires.

The comparison and sleep are atomic. In other words, if another thread writes a new value to *obj and then issues UMTX_OP_WAKE, the request is guaranteed to not miss the wakeup, which might otherwise happen between comparison and blocking.

The physical address of memory where the *obj variable is located, is used as a key to index sleeping threads.

The read of the current value of the *obj variable is not guarded by barriers. In particular, it is the user's duty to ensure the lock acquire and release memory semantics, if the UMTX_OP_WAIT and UMTX_OP_WAKE requests are used as a substrate for implementing a simple lock.

The request is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error.

Optionally, a timeout for the request may be specified.

UMTX_OP_WAKE

Wake the threads possibly sleeping due to UMTX_OP_WAIT. The arguments for the request are:

obj

Pointer to a variable, used as a key to find sleeping threads.

val

Up to val threads are woken up by this request. Specify INT_MAX to wake up all waiters.

UMTX_OP_MUTEX_TRYLOCK

Try to lock umutex. The arguments to the request are:

obj

Pointer to the umutex.

Operates same as the UMTX_OP_MUTEX_LOCK request, but returns EBUSY instead of sleeping if the lock cannot be obtained immediately.

UMTX_OP_MUTEX_LOCK

Lock umutex. The arguments to the request are:

obj

Pointer to the umutex.

Locking is performed by writing the current thread id into the m_owner word of the struct umutex. The write is atomic, preserves the UMUTEX_CONTESTED contention indicator, and provides the acquire barrier for lock entrance semantic.

If the lock cannot be obtained immediately because another thread owns the lock, the current thread is put to sleep, with UMUTEX_CONTESTED bit set before. Upon wake up, the lock conditions are re-tested.

The request adheres to the priority protection or inheritance protocol of the mutex, specified by the UMUTEX_PRIO_PROTECT or UMUTEX_PRIO_INHERIT flag, respectively.

Optionally, a timeout for the request may be specified.

A request with a timeout specified is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error. A request without timeout specified is always restarted after return from a signal handler.

UMTX_OP_MUTEX_UNLOCK

Unlock umutex. The arguments to the request are:

obj

Pointer to the umutex.

Unlocks the mutex, by writing UMUTEX_UNOWNED (zero) value into m_owner word of the struct umutex. The write is done with a release barrier, to provide lock leave semantic.

If there are threads sleeping in the sleep queue associated with the umutex, one thread is woken up. If more than one thread sleeps in the sleep queue, the UMUTEX_CONTESTED bit is set together with the write of the UMUTEX_UNOWNED value into m_owner.

The request adheres to the priority protection or inheritance protocol of the mutex, specified by the UMUTEX_PRIO_PROTECT or UMUTEX_PRIO_INHERIT flag, respectively. See description of the m_ceilings member of the struct umutex structure for additional details of the request operation on the priority protected protocol mutex.

UMTX_OP_SET_CEILING

Set ceiling for the priority protected umutex. The arguments to the request are:

obj

Pointer to the umutex.

val

New ceiling value.

uaddr

Address of a variable of type uint32_t. If not NULL and the update was successful, the previous ceiling value is written to the location pointed to by uaddr.

The request locks the umutex pointed to by the obj parameter, waiting for the lock if not immediately available. After the lock is obtained, the new ceiling value val is written to the m_ceilings[0] member of the struct umutex, after which the umutex is unlocked.

The locking does not adhere to the priority protect protocol, to conform to the POSIX requirements for the pthread_mutex_setprioceiling(3) interface.

UMTX_OP_CV_WAIT

Wait for a condition. The arguments to the request are:

obj

Pointer to the struct ucond.

val

Request flags, see below.

uaddr

Pointer to the umutex.

uaddr2

Optional pointer to a struct timespec for timeout specification.

The request must be issued by the thread owning the mutex pointed to by the uaddr argument. The c_hash_waiters member of the struct ucond, pointed to by the obj argument, is set to an arbitrary non-zero value, after which the uaddr mutex is unlocked (following the appropriate protocol), and the current thread is put to sleep on the sleep queue keyed by the obj argument. The operations are performed atomically. It is guaranteed to not miss a wakeup from UMTX_OP_CV_SIGNAL or UMTX_OP_CV_BROADCAST sent between mutex unlock and putting the current thread on the sleep queue.

Upon wakeup, if the timeout expired and no other threads are sleeping in the same sleep queue, the c_hash_waiters member is cleared. After wakeup, the uaddr umutex is not relocked.

The following flags are defined:

CVWAIT_ABSTIME

Timeout is absolute.

CVWAIT_CLOCKID

Clockid is provided.

Optionally, a timeout for the request may be specified. Unlike other requests, the timeout value is specified directly by a struct timespec, pointed to by the uaddr2 argument. If the CVWAIT_CLOCKID flag is provided, the timeout uses the clock from the c_clockid member of the struct ucond, pointed to by obj argument. Otherwise, CLOCK_REALTIME is used, regardless of the clock identifier possibly specified in the struct _umtx_time. If the CVWAIT_ABSTIME flag is supplied, the timeout specifies absolute time value, otherwise it denotes a relative time interval.

The request is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error.

UMTX_OP_CV_SIGNAL

Wake up one condition waiter. The arguments to the request are:

obj

Pointer to struct ucond.

The request wakes up at most one thread sleeping on the sleep queue keyed by the obj argument. If the woken up thread was the last on the sleep queue, the c_has_waiters member of the struct ucond is cleared.

UMTX_OP_CV_BROADCAST

Wake up all condition waiters. The arguments to the request are:

obj

Pointer to struct ucond.

The request wakes up all threads sleeping on the sleep queue keyed by the obj argument. The c_has_waiters member of the struct ucond is cleared.

UMTX_OP_WAIT_UINT

Same as UMTX_OP_WAIT, but the type of the variable pointed to by obj is u_int (a 32-bit integer).

UMTX_OP_RW_RDLOCK

Read-lock a struct rwlock lock. The arguments to the request are:

obj

Pointer to the lock (of type struct rwlock) to be read-locked.

val

Additional flags to augment locking behaviour. The valid flags in the val argument are:

URWLOCK_PREFER_READER

 

The request obtains the read lock on the specified struct rwlock by incrementing the count of readers in the rw_state word of the structure. If the URWLOCK_WRITE_OWNER bit is set in the word rw_state, the lock was granted to a writer which has not yet relinquished its ownership. In this case the current thread is put to sleep until it makes sense to retry.

If the URWLOCK_PREFER_READER flag is set either in the rw_flags word of the structure, or in the val argument of the request, the presence of the threads trying to obtain the write lock on the same structure does not prevent the current thread from trying to obtain the read lock. Otherwise, if the flag is not set, and the URWLOCK_WRITE_WAITERS flag is set in rw_state, the current thread does not attempt to obtain read-lock. Instead it sets the URWLOCK_READ_WAITERS in the rw_state word and puts itself to sleep on corresponding sleep queue. Upon wakeup, the locking conditions are re-evaluated.

Optionally, a timeout for the request may be specified.

The request is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error.

UMTX_OP_RW_WRLOCK

Write-lock a struct rwlock lock. The arguments to the request are:

obj

Pointer to the lock (of type struct rwlock) to be write-locked.

The request obtains a write lock on the specified struct rwlock, by setting the URWLOCK_WRITE_OWNER bit in the rw_state word of the structure. If there is already a write lock owner, as indicated by the URWLOCK_WRITE_OWNER bit being set, or there are read lock owners, as indicated by the read-lock counter, the current thread does not attempt to obtain the write-lock. Instead it sets the URWLOCK_WRITE_WAITERS in the rw_state word and puts itself to sleep on corresponding sleep queue. Upon wakeup, the locking conditions are re-evaluated.

Optionally, a timeout for the request may be specified.

The request is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error.

UMTX_OP_RW_UNLOCK

Unlock rwlock. The arguments to the request are:

obj

Pointer to the lock (of type struct rwlock) to be unlocked.

The unlock type (read or write) is determined by the current lock state. Note that the struct rwlock does not save information about the identity of the thread which acquired the lock.

If there are pending writers after the unlock, and the URWLOCK_PREFER_READER flag is not set in the rw_flags member of the *obj structure, one writer is woken up, selected as described in the SLEEP QUEUES subsection. If the URWLOCK_PREFER_READER flag is set, a pending writer is woken up only if there is no pending readers.

If there are no pending writers, or, in the case that the URWLOCK_PREFER_READER flag is set, then all pending readers are woken up by unlock.

UMTX_OP_WAIT_UINT_PRIVATE

Same as UMTX_OP_WAIT_UINT, but unconditionally select the process-private sleep queue.

UMTX_OP_WAKE_PRIVATE

Same as UMTX_OP_WAKE, but unconditionally select the process-private sleep queue.

UMTX_OP_MUTEX_WAIT

Wait for mutex availability. The arguments to the request are:

obj

Address of the mutex.

Similarly to the UMTX_OP_MUTEX_LOCK, put the requesting thread to sleep if the mutex lock cannot be obtained immediately. The UMUTEX_CONTESTED bit is set in the m_owner word of the mutex to indicate that there is a waiter, before the thread is added to the sleep queue. Unlike the UMTX_OP_MUTEX_LOCK request, the lock is not obtained.

The operation is not implemented for priority protected and priority inherited protocol mutexes.

Optionally, a timeout for the request may be specified.

A request with a timeout specified is not restartable. An unblocked signal delivered during the wait always results in sleep interruption and EINTR error. A request without a timeout automatically restarts if the signal disposition requested restart via the SA_RESTART flag in struct sigaction member sa_flags.

UMTX_OP_NWAKE_PRIVATE

Wake up a batch of sleeping threads. The arguments to the request are:

obj

Pointer to the array of pointers.

val

Number of elements in the array pointed to by obj.

For each element in the array pointed to by obj, wakes up all threads waiting on the private sleep queue with the key being the byte addressed by the array element.

UMTX_OP_MUTEX_WAKE

Check if a normal umutex is unlocked and wake up a waiter. The arguments for the request are:

obj

Pointer to the umutex.

If the m_owner word of the mutex pointed to by the obj argument indicates unowned mutex, which has its contention indicator bit UMUTEX_CONTESTED set, clear the bit and wake up one waiter in the sleep queue associated with the byte addressed by the obj, if any. Only normal mutexes are supported by the request. The sleep queue is always one for a normal mutex type.

This request is deprecated in favor of UMTX_OP_MUTEX_WAKE2 since mutexes using it cannot synchronize their own destruction. That is, the m_owner word has already been set to UMUTEX_UNOWNED when this request is made, so that another thread can lock, unlock and destroy the mutex (if no other thread uses the mutex afterwards). Clearing the UMUTEX_CONTESTED bit may then modify freed memory.

UMTX_OP_MUTEX_WAKE2

Check if a umutex is unlocked and wake up a waiter. The arguments for the request are:

obj

Pointer to the umutex.

val

The umutex flags.

The request does not read the m_flags member of the struct umutex; instead, the val argument supplies flag information, in particular, to determine the sleep queue where the waiters are found for wake up.

If the mutex is unowned, one waiter is woken up.

If the mutex memory cannot be accessed, all waiters are woken up.

If there is more than one waiter on the sleep queue, or there is only one waiter but the mutex is owned by a thread, the UMUTEX_CONTESTED bit is set in the m_owner word of the struct umutex.

UMTX_OP_SEM2_WAIT

Wait until semaphore is available. The arguments to the request are:

obj

Pointer to the semaphore (of type struct _usem2).

Put the requesting thread onto a sleep queue if the semaphore counter is zero. If the thread is put to sleep, the USEM_HAS_WAITERS bit is set in the _count word to indicate waiters. The function returns either due to _count indicating the semaphore is available (non-zero count due to post), or due to a wakeup. The return does not guarantee that the semaphore is available, nor does it consume the semaphore lock on successful return.

Optionally, a timeout for the request may be specified.

A request with non-absolute timeout value is not restartable. An unblocked signal delivered during such wait results in sleep interruption and EINTR error.

UMTX_OP_SEM2_WAKE

Wake up waiters on semaphore lock. The arguments to the request are:

obj

Pointer to the semaphore (of type struct _usem2).

The request wakes up one waiter for the semaphore lock. The function does not increment the semaphore lock count. If the USEM_HAS_WAITERS bit was set in the _count word, and the last sleeping thread was woken up, the bit is cleared.

UMTX_OP_SHM

Manage anonymous POSIX shared memory objects (see shm_open(2)), which can be attached to a byte of physical memory, mapped into the process address space. The objects are used to implement process-shared locks in libthr.

The val argument specifies the sub-request of the UMTX_OP_SHM request:

UMTX_SHM_CREAT

Creates the anonymous shared memory object, which can be looked up with the specified key uaddr. If the object associated with the uaddr key already exists, it is returned instead of creating a new object. The object's size is one page. On success, the file descriptor referencing the object is returned. The descriptor can be used for mapping the object using mmap(2), or for other shared memory operations.

UMTX_SHM_LOOKUP

Same as UMTX_SHM_CREATE request, but if there is no shared memory object associated with the specified key uaddr, an error is returned, and no new object is created.

UMTX_SHM_DESTROY

De-associate the shared object with the specified key uaddr. The object is destroyed after the last open file descriptor is closed and the last mapping for it is destroyed.

UMTX_SHM_ALIVE

Checks whether there is a live shared object associated with the supplied key uaddr. Returns zero if there is, and an error otherwise. This request is an optimization of the UMTX_SHM_LOOKUP request. It is cheaper when only the liveness of the associated object is asked for, since no file descriptor is installed in the process fd table on success.

The uaddr argument specifies the virtual address, which backing physical memory byte identity is used as a key for the anonymous shared object creation or lookup.

UMTX_OP_ROBUST_LISTS

Register the list heads for the current thread's robust mutex lists. The arguments to the request are:

val

Size of the structure passed in the uaddr argument.

uaddr

Pointer to the structure of type struct umtx_robust_lists_params.

The structure is defined as

struct umtx_robust_lists_params {
	uintptr_t	robust_list_offset;
	uintptr_t	robust_priv_list_offset;
	uintptr_t	robust_inact_offset;
};

The robust_list_offset member contains address of the first element in the list of locked robust shared mutexes. The robust_priv_list_offset member contains address of the first element in the list of locked robust private mutexes. The private and shared robust locked lists are split to allow fast termination of the shared list on fork, in the child.

The robust_inact_offset contains a pointer to the mutex which might be locked in nearby future, or might have been just unlocked. It is typically set by the lock or unlock mutex implementation code around the whole operation, since lists can be only changed race-free when the thread owns the mutex. The kernel inspects the robust_inact_offset in addition to walking the shared and private lists. Also, the mutex pointed to by robust_inact_offset is handled more loosely at the thread termination time, than other mutexes on the list. That mutex is allowed to be not owned by the current thread, in which case list processing is continued. See ROBUST UMUTEXES subsection for details.