From f36ca57f748bb0f8d2aa0d56422bf62b5c8fc9e9 Mon Sep 17 00:00:00 2001 From: Debian Haskell Group Date: Wed, 1 Apr 2015 11:35:10 +0000 Subject: [PATCH] bit-big-endian commit a4b1a43542b11d09dd3b603d82c5a0e99da67d74 Author: Austin Seipp 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 Signed-off-by: Austin Seipp Gbp-Pq: Name 64-bit-big-endian --- rts/STM.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/rts/STM.c b/rts/STM.c index 6bcb7ba0..c4dcb9f9 100644 --- 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 */ } -- 2.30.2