RFC: Data pattern buffer filling race condition fix

[Bart Van Assche <bvanassche@acm.org>]

On multicore non-x86 CPUs fio has been observed to frequently reports false
data verification failures with I/O engine libaio and I/O depths above one.
This is because of a race condition in the function fill_pattern(). The code
in that function only works correct if all CPUs of a multicore system
observe store instructions in the order they were issued. That is the case for
multicore x86 systems but not for all other CPU families, such as e.g. the
POWER CPU family.

As far as I can see this bug was introduced via commit
cbe8d7561cf6d81d741d87eb7940db2a111d2144 (July 14, 2010).

I'm posting this patch as an RFC since the fix is GCC-specific.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

diff --git a/verify.c b/verify.c
index ea1a911..3826198 100644
--- a/verify.c
+++ b/verify.c
@@ -31,18 +31,27 @@ void fill_pattern(struct thread_data *td, void *p, unsigned int len, struct io_u
 		fill_random_buf(p, len);
 		break;
 	case 1:
+#ifdef __GNUC__
+		__sync_synchronize();
+#endif
 		if (io_u->buf_filled_len >= len) {
 			dprint(FD_VERIFY, "using already filled verify pattern b=0 len=%u\n", len);
 			return;
 		}
 		dprint(FD_VERIFY, "fill verify pattern b=0 len=%u\n", len);
 		memset(p, td->o.verify_pattern[0], len);
+#ifdef __GNUC__
+		__sync_synchronize();
+#endif
 		io_u->buf_filled_len = len;
 		break;
 	default: {
 		unsigned int i = 0, size = 0;
 		unsigned char *b = p;
 
+#ifdef __GNUC__
+		__sync_synchronize();
+#endif
 		if (io_u->buf_filled_len >= len) {
 			dprint(FD_VERIFY, "using already filled verify pattern b=%d len=%u\n",
 					td->o.verify_pattern_bytes, len);
@@ -58,6 +67,9 @@ void fill_pattern(struct thread_data *td, void *p, unsigned int len, struct io_u
 			memcpy(b+i, td->o.verify_pattern, size);
 			i += size;
 		}
+#ifdef __GNUC__
+		__sync_synchronize();
+#endif
 		io_u->buf_filled_len = len;
 		break;
 		}