bit-big-endian

commit a4b1a43542b11d09dd3b603d82c5a0e99da67d74
Author: Austin Seipp <austin@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@linux.vnet.ibm.com>
    Signed-off-by: Austin Seipp <austin@well-typed.com>
Gbp-Pq: Name 64-bit-big-endian

diff --git a/rts/STM.c b/rts/STM.c
index 6bcb7ba0e04836962f4de5e6c7a4a5423f6716f9..c4dcb9f9869e9eee226ca22f3d92d61b6dbcffe3 100644 (file)
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -927,7 +927,7 @@ 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 */ }