[commit: ghc] master: Fix loop on 64bit Big-Endian platforms (#8134) (a4b1a43)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/a4b1a43542b11d09dd3b603d82c5a0e99da67d74/ghc

>---------------------------------------------------------------

commit a4b1a43542b11d09dd3b603d82c5a0e99da67d74
Author: Austin Seipp <austin at well-typed.com>
Date:   Fri Nov 1 22:17:01 2013 -0500

    Fix loop on 64bit Big-Endian platforms (#8134)
    
    This is a fun one.
    
    In the RTS, `cas` expects a pointer to StgWord which will translate to
    unsigned long (8 bytes under LP64.) But we had previously declared
    token_locked as *StgBool* - which evaluates to 'int' (4 bytes under
    LP64.) That means we fail to provide enough storage for the cas
    primitive, causing it to corrupt memory on a 64bit platform.
    
    Hilariously, this somehow did not affect little-endian platforms (ARM,
    x86, etc) before. That's because to clear our lock token, we would say:
    
        token_locked = 0;
    
    But because token_locked is 32bits technically, this only writes to
    half of the 64bit quantity. On a Big-Endian machine, this won't do
    anything. That is, token_locked starts as 0:
    
     / token_locked
     |
     v
     0x00000000
    
    and the first cas modifies the memory to:
    
     / valid    / corrupted
     |          |
     v          v
     0x00000000 0x00000001
    
    We then clear token_locked, but this doesn't change the corrupted 4
    bytes of memory. And then we try to lock the token again, spinning until
    it is released - clearly a deadlock.
    
    Related: Windows (amd64) doesn't follow LP64, but LLP64, where both
    int and long are 4 bytes, so this shouldn't change anything on these
    platforms.
    
    Thanks to Reid Barton for helping the diagnosis. Also, thanks to Jens
    Peterson who confirmed this also fixes building GHC on Fedora/ppc64 and
    Fedora/s390x.
    
    Authored-by: Gustavo Luiz Duarte <gustavold at linux.vnet.ibm.com>
    Signed-off-by: Austin Seipp <austin at well-typed.com>


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a4b1a43542b11d09dd3b603d82c5a0e99da67d74
 rts/STM.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rts/STM.c b/rts/STM.c
index e342ebf..bea0356 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -949,7 +949,7 @@ void stmPreGCHook (Capability *cap) {
 static volatile StgInt64 max_commits = 0;
 
 #if defined(THREADED_RTS)
-static volatile StgBool token_locked = FALSE;
+static volatile StgWord token_locked = FALSE;
 
 static void getTokenBatch(Capability *cap) {
   while (cas((void *)&token_locked, FALSE, TRUE) == TRUE) { /* nothing */ }