[MIPS] RTLX: Harden against compiler reordering and optimization.

RTLX communication is based on lock-free shared memory buffers.  It
happened to be working by luck so far but relies on the optimizer doing
certain optimizations but no reordering.

Fixed by inserting proper barriers in rtlx_read and rtlx_write, and careful
pointer dereferencing.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/rtlx.c

@@ -306,7 +306,7 @@ static inline void copy_from(void *dst, void *src, size_t count, int user)
 
 ssize_t rtlx_read(int index, void *buff, size_t count, int user)
 {
-	size_t fl = 0L;
+	size_t lx_write, fl = 0L;
 	struct rtlx_channel *lx;
 
 	if (rtlx == NULL)
@@ -314,23 +314,26 @@ ssize_t rtlx_read(int index, void *buff, size_t count, int user)
 
 	lx = &rtlx->channel[index];
 
+	smp_rmb();
+	lx_write = lx->lx_write;
+
 	/* find out how much in total */
 	count = min(count,
-		     (size_t)(lx->lx_write + lx->buffer_size - lx->lx_read)
+		     (size_t)(lx_write + lx->buffer_size - lx->lx_read)
 		     % lx->buffer_size);
 
 	/* then how much from the read pointer onwards */
-	fl = min( count, (size_t)lx->buffer_size - lx->lx_read);
+	fl = min(count, (size_t)lx->buffer_size - lx->lx_read);
 
-	copy_to(buff, &lx->lx_buffer[lx->lx_read], fl, user);
+	copy_to(buff, lx->lx_buffer + lx->lx_read, fl, user);
 
 	/* and if there is anything left at the beginning of the buffer */
-	if ( count - fl )
-		copy_to (buff + fl, lx->lx_buffer, count - fl, user);
+	if (count - fl)
+		copy_to(buff + fl, lx->lx_buffer, count - fl, user);
 
-	/* update the index */
-	lx->lx_read += count;
-	lx->lx_read %= lx->buffer_size;
+	smp_wmb();
+	lx->lx_read = (lx->lx_read + count) % lx->buffer_size;
+	smp_wmb();
 
 	return count;
 }
@@ -338,6 +341,7 @@ ssize_t rtlx_read(int index, void *buff, size_t count, int user)
 ssize_t rtlx_write(int index, void *buffer, size_t count, int user)
 {
 	struct rtlx_channel *rt;
+	size_t rt_read;
 	size_t fl;
 
 	if (rtlx == NULL)
@@ -345,24 +349,27 @@ ssize_t rtlx_write(int index, void *buffer, size_t count, int user)
 
 	rt = &rtlx->channel[index];
 
+	smp_rmb();
+	rt_read = rt->rt_read;
+
 	/* total number of bytes to copy */
 	count = min(count,
-		    (size_t)write_spacefree(rt->rt_read, rt->rt_write,
-					    rt->buffer_size));
+		    (size_t)write_spacefree(rt_read, rt->rt_write, rt->buffer_size));
 
 	/* first bit from write pointer to the end of the buffer, or count */
 	fl = min(count, (size_t) rt->buffer_size - rt->rt_write);
 
-	copy_from (&rt->rt_buffer[rt->rt_write], buffer, fl, user);
+	copy_from(rt->rt_buffer + rt->rt_write, buffer, fl, user);
 
 	/* if there's any left copy to the beginning of the buffer */
-	if( count - fl )
-		copy_from (rt->rt_buffer, buffer + fl, count - fl, user);
+	if (count - fl)
+		copy_from(rt->rt_buffer, buffer + fl, count - fl, user);
 
-	rt->rt_write += count;
-	rt->rt_write %= rt->buffer_size;
+	smp_wmb();
+	rt->rt_write = (rt->rt_write + count) % rt->buffer_size;
+	smp_wmb();
 
-	return(count);
+	return count;
 }