source: trunk/gcc/boehm-gc/doc/README.linux@ 3003

See README.alpha for Linux on DEC AXP info.

This file applies mostly to Linux/Intel IA32.  Ports to Linux on an M68K
and PowerPC are also integrated.  They should behave similarly, except that
the PowerPC port lacks incremental GC support, and it is unknown to what
extent the Linux threads code is functional.  See below for M68K specific
notes.

Incremental GC is supported on Intel IA32 and M68K.

Dynamic libraries are supported on an ELF system.  A static executable
should be linked with the gcc option "-Wl,-defsym,_DYNAMIC=0".

The collector appears to work with Linux threads.  We have seen
intermittent hangs in sem_wait.  So far we have been unable to reproduce
these unless the process was being debugged or traced.  Thus it's
possible that the only real issue is that the debugger loses
signals on rare occasions.

The garbage collector uses SIGPWR and SIGXCPU if it is used with
Linux threads.  These should not be touched by the client program.

To use threads, you need to abide by the following requirements:

1) You need to use LinuxThreads (which are included in libc6).

   The collector relies on some implementation details of the LinuxThreads
   package.  It is unlikely that this code will work on other
   pthread implementations (in particular it will *not* work with
   MIT pthreads).

2) You must compile the collector with -DGC_LINUX_THREADS and -D_REENTRANT
   specified in the Makefile.

3a) Every file that makes thread calls should define GC_LINUX_THREADS and 
   _REENTRANT and then include gc.h.  Gc.h redefines some of the
   pthread primitives as macros which also provide the collector with
   information it requires.

3b) A new alternative to (3a) is to build the collector and compile GC clients
   with -DGC_USE_LD_WRAP, and to link the final program with

   (for ld) --wrap read --wrap dlopen --wrap pthread_create \
            --wrap pthread_join --wrap pthread_detach \
            --wrap pthread_sigmask --wrap sleep

   (for gcc) -Wl,--wrap -Wl,read -Wl,--wrap -Wl,dlopen -Wl,--wrap \
             -Wl,pthread_create -Wl,--wrap -Wl,pthread_join -Wl,--wrap \
             -Wl,pthread_detach -Wl,--wrap -Wl,pthread_sigmask \
             -Wl,--wrap -Wl,sleep

   In any case, _REENTRANT should be defined during compilation.

4) Dlopen() disables collection during its execution.  (It can't run
   concurrently with the collector, since the collector looks at its
   data structures.  It can't acquire the allocator lock, since arbitrary
   user startup code may run as part of dlopen().)  Under unusual
   conditions, this may cause unexpected heap growth.

5) The combination of GC_LINUX_THREADS, REDIRECT_MALLOC, and incremental
   collection fails in seemingly random places.  This hasn't been tracked
   down yet, but is perhaps not completely astonishing.  The thread package
   uses malloc, and thus can presumably get SIGSEGVs while inside the
   package.  There is no real guarantee that signals are handled properly
   at that point.

6) Thread local storage may not be viewed as part of the root set by the
   collector.  This probably depends on the linuxthreads version.  For the
   time being, any collectable memory referenced by thread local storage should
   also be referenced from elsewhere, or be allocated as uncollectable.
   (This is really a bug that should be fixed somehow.)


M68K LINUX:
(From Richard Zidlicky)
The bad news is that it can crash every linux-m68k kernel on a 68040,
so an additional test is needed somewhere on startup. I have meanwhile
patches to correct the problem in 68040 buserror handler but it is not
yet in any standard kernel.

Here is a simple test program to detect whether the kernel has the
problem. It could be run as a separate check in configure or tested 
upon startup. If it fails (return !0) than mprotect can't be used
on that system.

/*
 * test for bug that may crash 68040 based Linux
 */

#include <sys/mman.h>
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>


char *membase;
int pagesize=4096;
int pageshift=12;
int x_taken=0;

int sighandler(int sig)
{
   mprotect(membase,pagesize,PROT_READ|PROT_WRITE);
   x_taken=1;
}

main()
{
  long l;

   signal(SIGSEGV,sighandler);
   l=(long)mmap(NULL,pagesize,PROT_READ,MAP_PRIVATE | MAP_ANON,-1,0);
  if (l==-1)
     {
       perror("mmap/malloc");
       abort();
     }
  membase=(char*)l;
    *(long*)(membase+sizeof(long))=123456789;
  if (*(long*)(membase+sizeof(long)) != 123456789 )
    {
      fprintf(stderr,"writeback failed !\n");
      exit(1);
    }
  if (!x_taken)
    {
      fprintf(stderr,"exception not taken !\n");
      exit(1);
    }
  fprintf(stderr,"vmtest Ok\n");
  exit(0);
}