libthr3

Usage

Library

library “libthr”

Overview

The libthr library provides a 1:1 implementation of the pthread(3) library interfaces for application threading. It has been optimized for use by applications expecting system scope thread semantics.

The library is tightly integrated with the run-time link editor ld-elf.so.1(1) and Standard C Library (libc, -lc); all three components must be built from the same source tree. Together, they constitute the base C runtime environment of FreeBSD, running on top of the FreeBSD kernel.

Mixing libc and libthr libraries from different versions of FreeBSD is not supported. The run-time linker ld-elf.so.1(1) has some code to ensure backward-compatibility with older versions of libthr.

The man page documents the quirks and tunables of the libthr. When linking with -lpthread, the run-time dependency libthr.so.3 is recorded in the produced object.

Mutex acquisition

A locked mutex (see pthread_mutex_lock(3)) is represented by a volatile variable of type lwpid_t, which records the global system identifier of the thread owning the lock. libthr performs a contested mutex acquisition in three stages, each of which is more resource-consuming than the previous. The first two stages are only applied for a mutex of PTHREAD_MUTEX_ADAPTIVE_NP type and PTHREAD_PRIO_NONE protocol (see pthread_mutexattr(3)).

First, on SMP systems, a spin loop is performed, where the library attempts to acquire the lock by atomic(9) operations. The loop count is controlled by the LIBPTHREAD_SPINLOOPS environment variable, with a default value of 2000.

If the spin loop was unable to acquire the mutex, a yield loop is executed, performing the same atomic(9) acquisition attempts as the spin loop, but each attempt is followed by a yield of the CPU time of the thread using the sched_yield(2) syscall. By default, the yield loop is not executed. This is controlled by the LIBPTHREAD_YIELDLOOPS environment variable.

If both the spin and yield loops failed to acquire the lock, the thread is taken off the CPU and put to sleep in the kernel with the _umtx_op(2) syscall. The kernel wakes up a thread and hands the ownership of the lock to the woken thread when the lock becomes available.

Thread stacks

Each thread is provided with a private user-mode stack area used by the C runtime. The size of the main (initial) thread stack is set by the kernel, and is controlled by the RLIMIT_STACK process resource limit (see getrlimit(2)).

By default, the main thread's stack size is equal to the value of RLIMIT_STACK for the process. If the LIBPTHREAD_SPLITSTACK_MAIN environment variable is present in the process environment (its value does not matter), the main thread's stack is reduced to 4MB on 64bit architectures, and to 2MB on 32bit architectures, when the threading library is initialized. The rest of the address space area which has been reserved by the kernel for the initial process stack is used for non-initial thread stacks in this case. The presence of the LIBPTHREAD_BIGSTACK_MAIN environment variable overrides LIBPTHREAD_SPLITSTACK_MAIN; it is kept for backward-compatibility.

The size of stacks for threads created by the process at run-time with the pthread_create(3) call is controlled by thread attributes: see pthread_attr(3), in particular, the pthread_attr_setstacksize(3), pthread_attr_setguardsize(3) and pthread_attr_setstackaddr(3) functions. If no attributes for the thread stack size are specified, the default non-initial thread stack size is 2MB for 64bit architectures, and 1MB for 32bit architectures.

Run-time settings

The following environment variables are recognized by libthr and adjust the operation of the library at run-time:

LIBPTHREAD_BIGSTACK_MAIN

Disables the reduction of the initial thread stack enabled by LIBPTHREAD_SPLITSTACK_MAIN.

LIBPTHREAD_SPLITSTACK_MAIN

Causes a reduction of the initial thread stack, as described in the section THREAD STACKS. This was the default behaviour of libthr before FreeBSD 11.0.

LIBPTHREAD_SPINLOOPS

The integer value of the variable overrides the default count of iterations in the spin loop of the mutex acquisition. The default count is 2000, set by the MUTEX_ADAPTIVE_SPINS constant in the libthr sources.

LIBPTHREAD_YIELDLOOPS

A non-zero integer value enables the yield loop in the process of the mutex acquisition. The value is the count of loop operations.

LIBPTHREAD_QUEUE_FIFO

The integer value of the variable specifies how often blocked threads are inserted at the head of the sleep queue, instead of its tail. Bigger values reduce the frequency of the FIFO discipline. The value must be between 0 and 31.

LIBPTHREAD_UMTX_MIN_TIMEOUT

The minimal amount of time, in nanoseconds, the thread is required to sleep for pthread operations specifying a timeout. If the operation requests a timeout less than the value provided, it is silently increased to the value. The value of zero means no minimum (default).

LIBPTHREAD_PSHARED_LOCK_DESTROY_IMMEDIATE_GC

The implementation of the process shared lock scans the mapped shared off-pages to detect locks destroyed by other processes, when the current process destroys a shared lock. This is done not more often than 25msec. Setting the environment variable causes the scan to run on each destroy.

The following sysctl MIBs affect the operation of the library:

kern.ipc.umtx_vnode_persistent

By default, a shared lock backed by a mapped file in memory is automatically destroyed on the last unmap of the corresponding file's page, which is allowed by POSIX. Setting the sysctl to 1 makes such a shared lock object persist until the vnode is recycled by the Virtual File System. Note that in case file is not opened and not mapped, the kernel might recycle it at any moment, making this sysctl less useful than it sounds.

kern.ipc.umtx_max_robust

The maximal number of robust mutexes allowed for one thread. The kernel will not unlock more mutexes than specified, see _umtx_op(2) for more details. The default value is large enough for most useful applications.

debug.umtx.robust_faults_verbose

A non zero value makes kernel emit some diagnostic when the robust mutexes unlock was prematurely aborted after detecting some inconsistency, as a measure to prevent memory corruption.

The RLIMIT_UMTXP limit (see getrlimit(2)) defines how many shared locks a given user may create simultaneously.

Interaction with run-time linker

On load, libthr installs interposing handlers into the hooks exported by libc. The interposers provide real locking implementation instead of the stubs for single-threaded processes in libc, cancellation support and some modifications to the signal operations.

libthr cannot be unloaded; the dlclose(3) function does not perform any action when called with a handle for libthr. One of the reasons is that the internal interposing of libc functions cannot be undone.

Signals

The implementation interposes the user-installed signal(3) handlers. This interposing is done to postpone signal delivery to threads which entered (libthr-internal) critical sections, where the calling of the user-provided signal handler is unsafe. An example of such a situation is owning the internal library lock. When a signal is delivered while the signal handler cannot be safely called, the call is postponed and performed until after the exit from the critical section. This should be taken into account when interpreting ktrace(1) logs.

The libthr library uses the SIGTHR signal for internal operations, in particular, for cancellation requests. This signal's masking and disposition is controlled by the library, and user programs should not try to modify them. The library interposes functions controlling signals to prevent inadvertent modifications and to guard portable code against exposure to SIGTHR.

Note: similarly, the SIGLIBRT signal is reserved for use by library “librt”, and should be not modified by users.

Process-shared synchronization objects

In the libthr implementation, user-visible types for all synchronization objects (e.g. pthread_mutex_t) are pointers to internal structures, allocated either by the corresponding pthread_<objtype>_init() method call, or implicitly on first use when a static initializer was specified. The initial implementation of process-private locking object used this model with internal allocation, and the addition of process-shared objects was done in a way that did not break the application binary interface.

For process-private objects, the internal structure is allocated using either malloc(3) or, for pthread_mutex_init(3), an internal memory allocator implemented in libthr. The internal allocator for mutexes is used to avoid bootstrap issues with many malloc(3) implementations which need working mutexes to function. The same allocator is used for thread-specific data, see pthread_setspecific(3), for the same reason.

For process-shared objects, the internal structure is created by first allocating a shared memory segment using _umtx_op(2) operation UMTX_OP_SHM, and then mapping it into process address space with mmap(2) with the MAP_SHARED flag. The POSIX standard requires that:

only the process-shared synchronization object itself can be used for
performing synchronization.  It need not be referenced at the address
used to initialize it (that is, another mapping of the same object can
be used).

With the FreeBSD implementation, process-shared objects require initialization in each process that use them. In particular, if you map the shared memory containing the user portion of a process-shared object already initialized in different process, locking functions do not work on it.

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 fork(2) anyway.