Add a new function dllockinit() for registering thread locking

functions to be used by the dynamic linker.  This can be called by
threads packages at start-up time.  I will add the call to libc_r
soon.

Also add a default locking method that is used up until dllockinit()
is called.  The default method works by blocking SIGVTALRM, SIGPROF,
and SIGALRM in critical sections.  It is based on the observation
that most user-space threads packages implement thread preemption
with one of these signals (usually SIGVTALRM).

The dynamic linker has never been reentrant, but it became less
reentrant in revision 1.34 of "src/libexec/rtld-elf/rtld.c".
Starting with that revision, multiple threads each doing lazy
binding could interfere with each other.  The usual symptom was
that a symbol was falsely reported as undefined at start-up time.
It was rare but not unseen.  This commit fixes it.

1 files changed, 109 insertions, 0 deletions

diff --git a/lib/libc/gen/dllockinit.3 b/lib/libc/gen/dllockinit.3
new file mode 100644
index 0000000..facb18b
--- /dev/null
+++ b/lib/libc/gen/dllockinit.3
@@ -0,0 +1,109 @@
+.\"
+.\" Copyright (c) 1999 John D. Polstra
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd December 26, 1999
+.Os FreeBSD
+.Dt DLLOCKINIT 3
+.Sh NAME
+.Nm dllockinit
+.Nd register thread locking methods with the dynamic linker
+.Sh SYNOPSIS
+.Fd #include <dlfcn.h>
+.Ft void
+.Fn dllockinit "const void *context" "void *(*lock_create)(void *context)" \
+"void (*rlock_acquire)(void *lock)" "void (*wlock_acquire)(void *lock)" \
+"void (*lock_release)(void *lock)" "void (*lock_destroy)(void *lock)" \
+"void (*context_destroy)(void *context)"
+.Sh DESCRIPTION
+Threads packages can call
+.Nm
+at initialization time to register locking functions for the dynamic
+linker to use.  This enables the dynamic linker to prevent multiple
+threads from entering its critical sections simultaneously.
+.Pp
+The
+.Fa context
+parameter specifies an opaque context for creating locks.  The
+dynamic linker will pass it to the
+.Fa lock_create
+function when creating the locks it needs.  When the dynamic linker
+is permanently finished using the locking functions (e.g., if the
+program makes a subsequent call to
+.Nm
+to register new locking functions) it will call
+.Fa context_destroy
+to destroy the context.
+.Pp
+The
+.Fa lock_create
+parameter specifies a function for creating a read/write lock.  It
+must return a pointer to the new lock.
+.Pp
+The
+.Fa rlock_acquire
+and
+.Fa wlock_acquire
+parameters specify functions which lock a lock for reading or
+writing, respectively.  The
+.Fa lock_release
+parameter specifies a function which unlocks a lock.  Each of these
+functions is passed a pointer to the lock.
+.Pp
+The
+.Fa lock_destroy
+parameter specifies a function to destroy a lock.  It may be
+.Dv NULL
+if locks do not need to be destroyed.  The
+.Fa context_destroy
+specifies a function to destroy the context.  It may be
+.Dv NULL
+if the context does not need to be destroyed.
+.Pp
+Before
+.Nm
+is called, the dynamic linker protects its critical sections by
+blocking the
+.Dv SIGVTALRM ,
+.Dv SIGPROF ,
+and
+.Dv SIGALRM
+signals.  This is sufficient for many application level threads
+packages, which typically use one of these signals to implement
+preemption.  An application which has registered its own locking
+methods with 
+.Nm
+can restore the default locking by calling
+.Nm
+with all arguments
+.Dv NULL .
+.Sh SEE ALSO
+.Xr rtld 1 ,
+.Xr signal 3
+.Sh HISTORY
+The
+.Nm
+function first appeared in FreeBSD 4.0.