295 lines
9.1 KiB
Groff
295 lines
9.1 KiB
Groff
.\" Copyright (c) 2005 Robert N. M. Watson
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.\" Copyright (c) 2014,2015 The FreeBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" Part of this documentation was written by
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.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
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.\" from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd May 17, 2016
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.Dt LIBTHR 3
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.Os
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.Sh NAME
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.Nm libthr
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.Nd "1:1 POSIX threads library"
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.Sh LIBRARY
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.Lb libthr
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.Sh SYNOPSIS
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.In pthread.h
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.Sh DESCRIPTION
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The
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.Nm
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library provides a 1:1 implementation of the
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.Xr pthread 3
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library interfaces for application threading.
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It
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has been optimized for use by applications expecting system scope thread
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semantics.
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.Pp
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The library is tightly integrated with the run-time link editor
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.Xr ld-elf.so.1 1
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and
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.Lb libc ;
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all three components must be built from the same source tree.
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Mixing
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.Li libc
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and
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.Nm
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libraries from different versions of
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.Fx
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is not supported.
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The run-time linker
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.Xr ld-elf.so.1 1
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has some code to ensure backward-compatibility with older versions of
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.Nm .
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.Pp
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The man page documents the quirks and tunables of the
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.Nm .
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When linking with
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.Li -lpthread ,
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the run-time dependency
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.Li libthr.so.3
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is recorded in the produced object.
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.Sh MUTEX ACQUISITION
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A locked mutex (see
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.Xr pthread_mutex_lock 3 )
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is represented by a volatile variable of type
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.Dv lwpid_t ,
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which records the global system identifier of the thread
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owning the lock.
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.Nm
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performs a contested mutex acquisition in three stages, each of which
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is more resource-consuming than the previous.
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The first two stages are only applied for a mutex of
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.Dv PTHREAD_MUTEX_ADAPTIVE_NP
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type and
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.Dv PTHREAD_PRIO_NONE
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protocol (see
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.Xr pthread_mutexattr 3 ) .
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.Pp
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First, on SMP systems, a spin loop
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is performed, where the library attempts to acquire the lock by
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.Xr atomic 9
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operations.
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The loop count is controlled by the
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.Ev LIBPTHREAD_SPINLOOPS
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environment variable, with a default value of 2000.
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.Pp
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If the spin loop
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was unable to acquire the mutex, a yield loop
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is executed, performing the same
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.Xr atomic 9
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acquisition attempts as the spin loop,
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but each attempt is followed by a yield of the CPU time
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of the thread using the
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.Xr sched_yield 2
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syscall.
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By default, the yield loop
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is not executed.
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This is controlled by the
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.Ev LIBPTHREAD_YIELDLOOPS
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environment variable.
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.Pp
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If both the spin and yield loops
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failed to acquire the lock, the thread is taken off the CPU and
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put to sleep in the kernel with the
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.Xr _umtx_op 2
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syscall.
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The kernel wakes up a thread and hands the ownership of the lock to
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the woken thread when the lock becomes available.
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.Sh THREAD STACKS
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Each thread is provided with a private user-mode stack area
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used by the C runtime.
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The size of the main (initial) thread stack is set by the kernel, and is
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controlled by the
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.Dv RLIMIT_STACK
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process resource limit (see
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.Xr getrlimit 2 ) .
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.Pp
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By default, the main thread's stack size is equal to the value of
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.Dv RLIMIT_STACK
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for the process.
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If the
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.Ev LIBPTHREAD_SPLITSTACK_MAIN
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environment variable is present in the process environment
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(its value does not matter),
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the main thread's stack is reduced to 4MB on 64bit architectures, and to
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2MB on 32bit architectures, when the threading library is initialized.
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The rest of the address space area which has been reserved by the
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kernel for the initial process stack is used for non-initial thread stacks
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in this case.
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The presence of the
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.Ev LIBPTHREAD_BIGSTACK_MAIN
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environment variable overrides
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.Ev LIBPTHREAD_SPLITSTACK_MAIN ;
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it is kept for backward-compatibility.
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.Pp
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The size of stacks for threads created by the process at run-time
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with the
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.Xr pthread_create 3
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call is controlled by thread attributes: see
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.Xr pthread_attr 3 ,
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in particular, the
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.Xr pthread_attr_setstacksize 3 ,
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.Xr pthread_attr_setguardsize 3
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and
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.Xr pthread_attr_setstackaddr 3
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functions.
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If no attributes for the thread stack size are specified, the default
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non-initial thread stack size is 2MB for 64bit architectures, and 1MB
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for 32bit architectures.
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.Sh RUN-TIME SETTINGS
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The following environment variables are recognized by
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.Nm
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and adjust the operation of the library at run-time:
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.Bl -tag -width "Ev LIBPTHREAD_SPLITSTACK_MAIN"
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.It Ev LIBPTHREAD_BIGSTACK_MAIN
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Disables the reduction of the initial thread stack enabled by
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.Ev LIBPTHREAD_SPLITSTACK_MAIN .
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.It Ev LIBPTHREAD_SPLITSTACK_MAIN
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Causes a reduction of the initial thread stack, as described in the
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section
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.Sx THREAD STACKS .
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This was the default behaviour of
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.Nm
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before
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.Fx 11.0 .
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.It Ev LIBPTHREAD_SPINLOOPS
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The integer value of the variable overrides the default count of
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iterations in the
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.Li spin loop
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of the mutex acquisition.
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The default count is 2000, set by the
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.Dv MUTEX_ADAPTIVE_SPINS
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constant in the
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.Nm
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sources.
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.It Ev LIBPTHREAD_YIELDLOOPS
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A non-zero integer value enables the yield loop
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in the process of the mutex acquisition.
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The value is the count of loop operations.
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.It Ev LIBPTHREAD_QUEUE_FIFO
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The integer value of the variable specifies how often blocked
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threads are inserted at the head of the sleep queue, instead of its tail.
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Bigger values reduce the frequency of the FIFO discipline.
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The value must be between 0 and 255.
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.Pp
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.El
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The following
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.Dv sysctl
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MIBs affect the operation of the library:
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.Bl -tag -width "Dv debug.umtx.robust_faults_verbose"
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.It Dv kern.ipc.umtx_vnode_persistent
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By default, a shared lock backed by a mapped file in memory is
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automatically destroyed on the last unmap of the corresponding file's page,
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which is allowed by POSIX.
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Setting the sysctl to 1 makes such a shared lock object persist until
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the vnode is recycled by the Virtual File System.
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Note that in case file is not opened and not mapped, the kernel might
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recycle it at any moment, making this sysctl less useful than it sounds.
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.It Dv kern.ipc.umtx_max_robust
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The maximal number of robust mutexes allowed for one thread.
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The kernel will not unlock more mutexes than specified, see
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.Xr _umtx_op
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for more details.
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The default value is large enough for most useful applications.
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.It Dv debug.umtx.robust_faults_verbose
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A non zero value makes kernel emit some diagnostic when the robust
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mutexes unlock was prematurely aborted after detecting some inconsistency,
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as a measure to prevent memory corruption.
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.El
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.Pp
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The
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.Dv RLIMIT_UMTXP
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limit (see
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.Xr getrlimit 2 )
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defines how many shared locks a given user may create simultaneously.
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.Sh INTERACTION WITH RUN-TIME LINKER
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On load,
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.Nm
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installs interposing handlers into the hooks exported by
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.Li libc .
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The interposers provide real locking implementation instead of the
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stubs for single-threaded processes in
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.Li libc ,
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cancellation support and some modifications to the signal operations.
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.Pp
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.Nm
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cannot be unloaded; the
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.Xr dlclose 3
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function does not perform any action when called with a handle for
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.Nm .
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One of the reasons is that the internal interposing of
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.Li libc
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functions cannot be undone.
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.Sh SIGNALS
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The implementation interposes the user-installed
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.Xr signal 3
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handlers.
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This interposing is done to postpone signal delivery to threads which
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entered (libthr-internal) critical sections, where the calling
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of the user-provided signal handler is unsafe.
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An example of such a situation is owning the internal library lock.
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When a signal is delivered while the signal handler cannot be safely
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called, the call is postponed and performed until after the exit from
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the critical section.
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This should be taken into account when interpreting
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.Xr ktrace 1
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logs.
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.Sh SEE ALSO
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.Xr ktrace 1 ,
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.Xr ld-elf.so.1 1 ,
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.Xr getrlimit 2 ,
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.Xr errno 2 ,
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.Xr thr_exit 2 ,
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.Xr thr_kill 2 ,
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.Xr thr_kill2 2 ,
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.Xr thr_new 2 ,
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.Xr thr_self 2 ,
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.Xr thr_set_name 2 ,
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.Xr _umtx_op 2 ,
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.Xr dlclose 3 ,
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.Xr dlopen 3 ,
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.Xr getenv 3 ,
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.Xr pthread_attr 3 ,
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.Xr pthread_attr_setstacksize 3 ,
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.Xr pthread_create 3 ,
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.Xr signal 3 ,
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.Xr atomic 9
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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library
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was originally created by
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.An Jeff Roberson Aq Mt jeff@FreeBSD.org ,
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and enhanced by
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.An Jonathan Mini Aq Mt mini@FreeBSD.org
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and
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.An Mike Makonnen Aq Mt mtm@FreeBSD.org .
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It has been substantially rewritten and optimized by
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.An David Xu Aq Mt davidxu@FreeBSD.org .
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