Date: Fri, 8 Oct 2021 00:43:15 GMT From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org Subject: git: 8e915bdea540 - stable/13 - libthr(3): explain some internals of the locks implementation Message-ID: <202110080043.1980hFer069432@gitrepo.freebsd.org>
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The branch stable/13 has been updated by kib: URL: https://cgit.FreeBSD.org/src/commit/?id=8e915bdea5406ff2266a18945829407feb465f63 commit 8e915bdea5406ff2266a18945829407feb465f63 Author: Konstantin Belousov <kib@FreeBSD.org> AuthorDate: 2021-10-01 01:17:02 +0000 Commit: Konstantin Belousov <kib@FreeBSD.org> CommitDate: 2021-10-08 00:42:38 +0000 libthr(3): explain some internals of the locks implementation (cherry picked from commit f5b9747075a9b489226e2a911f8a1597f4b9d072) --- lib/libthr/libthr.3 | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 61 insertions(+), 2 deletions(-) diff --git a/lib/libthr/libthr.3 b/lib/libthr/libthr.3 index 2b41187cdb7c..3018a6f20b86 100644 --- 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 +259,65 @@ the critical section. 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 (e.g. pthread_mutex_t) +are pointers to internal structures, allocated either by the corresponding +.Fn 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. +.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. +The 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 a 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 the +.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 initialize it (that is, another mapping of the same object can +be used). +.Ed +.Pp +With the +.Fx +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. +.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 ,
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