Minor multi-threading patches for VMS.

[p5sagit/p5-mst-13.2.git] / README.threads

Building

Omit the -e from your ./Configure arguments. For example, use
    ./Configure -drs
When it offers to let you change config.sh, do so. If you already
have a config.sh then you can edit it and do
    ./Configure -S
to propagate the required changes.
In ccflags, insert -DUSE_THREADS (and probably -DDEBUGGING since
that's what I've been building with). Also insert any other
arguments in there that your compiler needs to use POSIX threads.
Change optimize to -g to give you better debugging information.
Include any necessary explicit libraries in libs and change
ldflags if you need any linker flags instead or as well.

More explicitly, for Linux (when using the standard kernel-threads
based LinuxThreads library):
    Add -DUSE_THREADS -D_REENTRANT -DDEBUGGING to ccflags and cppflags
    Add -lpthread to libs
    Change optimize to -g
For Digital Unix 4.x:
    Add -pthread -DUSE_THREADS -DDEBUGGING to ccflags
    Add -DUSE_THREADS -DDEBUGGING to cppflags
    Add -pthread to ldflags
    Change optimize to -g
    Maybe add -lpthread -lc_r to lddlflags
    For some reason, the extra includes for pthreads make Digital UNIX
    complain fatally about the sbrk() delcaration in perl's malloc.c
    so use the native malloc as follows:
    Change usemymalloc to n
    Zap mallocobj and mallocsrc (foo='')
    Change d_mymalloc to undef


Now you can do a
    make perl
For Digital UNIX, it will get as far as building miniperl and then
bomb out buidling DynaLoader when MakeMaker tries to find where
perl is. This seems to be a problem with backticks/system when
threading is in. A minimal failing example is
    perl -e 'eval q($foo = 0); system("echo foo")'
which doesn't echo anything. The resulting ext/DynaLoader/Makefile
will have lines
    PERL = 0
    FULLPERL = 0
Change them to be the pathnames of miniperl and perl respectively
(the ones in your perl build directory). The resume the make with
    make perl
This time it should manage to build perl. If not, try some cutting
and pasting to compile and link things manually. Be careful when
building extensions that your ordinary perl doesn't end up making
a Makefile without the correct pthreads compiler options.

Building the Thread extension

Build it away from the perl tree in the usual way. Set your PATH
environment variable to have your perl build directory first and
set PERL5LIB to be /your/perl/build/directory/lib (without those,
I had problems where the config information from the ordinary perl
on the system would end up in the Makefile). Then
    perl Makefile.PL PERL_SRC=/your/perl/build/directory
    make
On Digital UNIX, you'll probably have to fix the "PERL = 0" and
"FULLPERL = 0" lines in the generated Makefile as for DynaLoader.

Then you can try some of the tests with
    perl -Mblib create.t
    perl -Mblib join.t
    perl -Mblib lock.t
    perl -Mblib unsync.t
    perl -Mblib unsync2.t
    perl -Mblib unsync3.t
    perl -Mblib io.t
The io one leaves a thread reading from the keyboard on stdin so
as the ping messages appear you can type lines and see them echoed.

Try running the main perl test suite too. There are known
failures for po/misc test 45 (tries to do local(@_) but @_ is
now lexical) and some tests involving backticks/system/fork
may or may not work. Under Linux, many tests appear to fail
when run under the test harness but work fine when invoked
manually.


Bugs

* Thread states (DETACHED, JOINED etc.) and perl's idea of what's
  in scope and out of scope aren't properly integrated. Expect
  segaults and hangs when thread objects whose threads have ended
  go out of scope (e.g. at program exit).

* cond.t hasn't been redone since condition variable changed.

* FAKE_THREADS should produce a working perl but the Thread
extension won't build with it yet.

* There's a known memory leak (curstack isn't freed at the end
of each thread because it causes refcount problems that I
haven't tracked down yet) and there are very probably others too.

* There are still races where bugs show up under contention.

* Need to document "lock", Thread.pm, Queue.pm, ...

* Plenty of others


Debugging

Use the -DL command-line option to turn on debugging of the
multi-threading code. Under Linux, that also turns on a quick
hack I did to grab a bit of extra information from segfaults.
If you have a fancier gdb/threads setup than I do then you'll
have to delete the lines in perl.c which say
    #if defined(DEBUGGING) && defined(USE_THREADS) && defined(__linux__)
        DEBUG_L(signal(SIGSEGV, (void(*)(int))catch_sigsegv););
    #endif


Background

Some old globals (e.g. stack_sp, op) and some old per-interpreter
variables (e.g. tmps_stack, cxstack) move into struct thread.
All fields of struct thread (apart from a few only applicable to
FAKE_THREADS) are of the form Tfoo. For example, stack_sp becomes
the field Tstack_sp of struct thread. For those fields which moved
from original perl, thread.h does
    #define foo (thr->Tfoo)
This means that all functions in perl which need to use one of these
fields need an (automatic) variable thr which points at the current
thread's struct thread. For pp_foo functions, it is passed around as
an argument, for other functions they do
    dTHR;
which declares and initialises thr from thread-specific data
via pthread_getspecific. If a function fails to compile with an
error about "no such variable thr", it probably just needs a dTHR
at the top.


Fake threads

For FAKE_THREADS, thr is a global variable and perl schedules threads
by altering thr in between appropriate ops. The next and prev fields
of struct thread keep all fake threads on a doubly linked list and
the next_run and prev_run fields keep all runnable threads on a
doubly linked list. Mutexes are stubs for FAKE_THREADS. Condition
variables are implemented as a list of waiting threads.


Mutexes and condition variables

The API is via macros MUTEX_{INIT,LOCK,UNLOCK,DESTROY} and
COND_{INIT,WAIT,SIGNAL,BROADCAST,DESTROY}. For POSIX threads,
perl mutexes and condition variables correspond to POSIX ones.
For FAKE_THREADS, mutexes are stubs and condition variables are
implmented as lists of waiting threads. For FAKE_THREADS, a thread
waits on a condition variable by removing itself from the runnable
list, calling SCHEDULE to change thr to the next appropriate
runnable thread and returning op (i.e. the new threads next op).
This means that fake threads can only block while in PP code.
A PP function which contains a COND_WAIT must be prepared to
handle such restarts and can use the field "private" of struct
thread to record its state. For fake threads, COND_SIGNAL and
COND_BROADCAST work by putting back all the threads on the
condition variables list into the run queue. Note that a mutex
must *not* be held while returning from a PP function.

Perl locks are a condpair_t structure (a triple of a mutex, a
condtion variable and an owner thread field) attached by 'm'
magic to any SV. pp_lock locks such an object by waiting on the
condition variable until the owner field is zero and then setting
the owner field to its own thread pointer. The lock is recursive
so if the owner field already matches the current thread then
pp_lock returns straight away. If the owner field has to be filled
in then unlock_condpair is queued as an end-of-block destructor and
that function zeroes out the owner field, releasing the lock.


Malcolm Beattie
mbeattie@sable.ox.ac.uk
9 September 1997