Fix ringbuffer thread safety on ARM. Fix #715 #388 (#886)

* fix ringbuffer thread safety on ARM. fix #715 #388

This patch addresses the thread safety problem of `jack_ringbuffer_t`
mentioned in #715 and #388. The overbound read bug caused by this problem
is impossible to reproduce on x86 due to its strong memory ordering, but
it is a problem on ARM and other weakly ordered architectures.

Basically, the main problem is that, on a weakly ordered architecture,
it is possible that the pointer increment after `memcpy` becomes visible
to the other thread before `memcpy` finishes:

	memcpy (&(rb->buf[rb->write_ptr]), src, n1);
	// vvv can be visible to reading thread before memcpy finishes
	rb->write_ptr = (rb->write_ptr + n1) & rb->size_mask;

If this happens, the other thread can read the remaining garbage values
in `rb->buf` due to be overwritten by the unfinished `memcpy`.

To fix this, an explicit pair of release/acquire memory fences [1] is
used to ensure the copy on the other thread *happens after* the `memcpy`
finishes so no garbage values can be read.

[1]: https://preshing.com/20130922/acquire-and-release-fences/

* remove volatile qualifier on ringbuf r/w pointers

The volatile constraints are excess when compiler barriers are present.
It generates unnecessary `mov` instructions when pointers aren't going
to be updated.

* simplify read/write space calculations

This optimization is possible because the buffer size is always a power
of 2. See [1] for details.

[1]: drobilla/zix#1 (comment)

* move acq fences to separate lines

common/jack/ringbuffer.h

@@ -50,8 +50,8 @@ jack_ringbuffer_data_t ;
 
 typedef struct {
     char	*buf;
-    volatile size_t write_ptr;
-    volatile size_t read_ptr;
+    size_t	write_ptr;
+    size_t	read_ptr;
     size_t	size;
     size_t	size_mask;
     int	mlocked;

common/ringbuffer.c

@@ -27,6 +27,35 @@
 #endif /* USE_MLOCK */
 #include "JackCompilerDeps.h"
 
+/* Portable definitions for acquire and release fences */
+#if defined(_MSC_VER)
+	#if defined(_M_AMD64) || defined(_M_IX86) || defined(_M_X64)
+		/* Only compiler fences are needed on x86. In fact, GCC
+		 * will generate no instructions for acq/rel fences on
+		 * x86 */
+		#include <intrin.h>
+		#define JACK_ACQ_FENCE() _ReadBarrier()
+		#define JACK_REL_FENCE() _WriteBarrier()
+	#else
+		/* Use full memory fence for non-x86 systems with msvc */
+		#include <windows.h>
+		#define JACK_ACQ_FENCE() MemoryBarrier()
+		#define JACK_REL_FENCE() MemoryBarrier()
+	#endif
+#elif defined(__GNUC__)
+	#ifdef __ATOMIC_ACQUIRE
+		#define JACK_ACQ_FENCE() __atomic_thread_fence(__ATOMIC_ACQUIRE)
+		#define JACK_REL_FENCE() __atomic_thread_fence(__ATOMIC_RELEASE)
+	#else
+		/* Fallback to the legacy __sync builtin (full memory fence) */
+		#define JACK_ACQ_FENCE() __sync_synchronize()
+		#define JACK_REL_FENCE() __sync_synchronize()
+	#endif
+#else
+	#define JACK_ACQ_FENCE()
+	#define JACK_REL_FENCE()
+#endif
+
 typedef struct {
     char *buf;
     size_t len;
@@ -35,8 +64,8 @@ jack_ringbuffer_data_t ;
 
 typedef struct {
     char	*buf;
-    volatile size_t write_ptr;
-    volatile size_t read_ptr;
+    size_t	write_ptr;
+    size_t	read_ptr;
     size_t	size;
     size_t	size_mask;
     int	mlocked;
@@ -126,7 +155,7 @@ jack_ringbuffer_reset (jack_ringbuffer_t * rb)
 {
 	rb->read_ptr = 0;
 	rb->write_ptr = 0;
-    memset(rb->buf, 0, rb->size);
+	memset(rb->buf, 0, rb->size);
 }
 
 /* Reset the read and write pointers to zero. This is not thread
@@ -152,13 +181,10 @@ jack_ringbuffer_read_space (const jack_ringbuffer_t * rb)
 	size_t w, r;
 
 	w = rb->write_ptr;
+	JACK_ACQ_FENCE();
 	r = rb->read_ptr;
 
-	if (w > r) {
-		return w - r;
-	} else {
-		return (w - r + rb->size) & rb->size_mask;
-	}
+	return (w - r) & rb->size_mask;
 }
 
 /* Return the number of bytes available for writing.  This is the
@@ -172,14 +198,9 @@ jack_ringbuffer_write_space (const jack_ringbuffer_t * rb)
 
 	w = rb->write_ptr;
 	r = rb->read_ptr;
+	JACK_ACQ_FENCE();
 
-	if (w > r) {
-		return ((r - w + rb->size) & rb->size_mask) - 1;
-	} else if (w < r) {
-		return (r - w) - 1;
-	} else {
-		return rb->size - 1;
-	}
+	return (r - w - 1) & rb->size_mask;
 }
 
 /* The copying data reader.  Copy at most `cnt' bytes from `rb' to
@@ -199,6 +220,8 @@ jack_ringbuffer_read (jack_ringbuffer_t * rb, char *dest, size_t cnt)
 
 	to_read = cnt > free_cnt ? free_cnt : cnt;
 
+	/* note: relaxed load here, rb->read_ptr cannot be
+	 * modified from writing thread  */
 	cnt2 = rb->read_ptr + to_read;
 
 	if (cnt2 > rb->size) {
@@ -210,10 +233,12 @@ jack_ringbuffer_read (jack_ringbuffer_t * rb, char *dest, size_t cnt)
 	}
 
 	memcpy (dest, &(rb->buf[rb->read_ptr]), n1);
+	JACK_REL_FENCE(); /* ensure pointer increment happens after copy */
 	rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask;
 
 	if (n2) {
 		memcpy (dest + n1, &(rb->buf[rb->read_ptr]), n2);
+		JACK_REL_FENCE(); /* ensure pointer increment happens after copy */
 		rb->read_ptr = (rb->read_ptr + n2) & rb->size_mask;
 	}
 
@@ -278,6 +303,8 @@ jack_ringbuffer_write (jack_ringbuffer_t * rb, const char *src, size_t cnt)
 
 	to_write = cnt > free_cnt ? free_cnt : cnt;
 
+	/* note: relaxed load here, rb->write_ptr cannot be
+	 * modified from reading thread  */
 	cnt2 = rb->write_ptr + to_write;
 
 	if (cnt2 > rb->size) {
@@ -289,10 +316,12 @@ jack_ringbuffer_write (jack_ringbuffer_t * rb, const char *src, size_t cnt)
 	}
 
 	memcpy (&(rb->buf[rb->write_ptr]), src, n1);
+	JACK_REL_FENCE(); /* ensure pointer increment happens after copy */
 	rb->write_ptr = (rb->write_ptr + n1) & rb->size_mask;
 
 	if (n2) {
 		memcpy (&(rb->buf[rb->write_ptr]), src + n1, n2);
+		JACK_REL_FENCE(); /* ensure pointer increment happens after copy */
 		rb->write_ptr = (rb->write_ptr + n2) & rb->size_mask;
 	}
 
@@ -305,6 +334,7 @@ LIB_EXPORT void
 jack_ringbuffer_read_advance (jack_ringbuffer_t * rb, size_t cnt)
 {
 	size_t tmp = (rb->read_ptr + cnt) & rb->size_mask;
+	JACK_REL_FENCE(); /* ensure pointer increment happens after copy (by user) */
 	rb->read_ptr = tmp;
 }
 
@@ -314,6 +344,7 @@ LIB_EXPORT void
 jack_ringbuffer_write_advance (jack_ringbuffer_t * rb, size_t cnt)
 {
 	size_t tmp = (rb->write_ptr + cnt) & rb->size_mask;
+	JACK_REL_FENCE(); /* ensure pointer increment happens after copy (by user) */
 	rb->write_ptr = tmp;
 }
 
@@ -328,17 +359,10 @@ jack_ringbuffer_get_read_vector (const jack_ringbuffer_t * rb,
 {
 	size_t free_cnt;
 	size_t cnt2;
-	size_t w, r;
+	size_t r;
 
-	w = rb->write_ptr;
 	r = rb->read_ptr;
-
-	if (w > r) {
-		free_cnt = w - r;
-	} else {
-		free_cnt = (w - r + rb->size) & rb->size_mask;
-	}
-
+	free_cnt = jack_ringbuffer_read_space(rb);
 	cnt2 = r + free_cnt;
 
 	if (cnt2 > rb->size) {
@@ -372,19 +396,10 @@ jack_ringbuffer_get_write_vector (const jack_ringbuffer_t * rb,
 {
 	size_t free_cnt;
 	size_t cnt2;
-	size_t w, r;
+	size_t w;
 
 	w = rb->write_ptr;
-	r = rb->read_ptr;
-
-	if (w > r) {
-		free_cnt = ((r - w + rb->size) & rb->size_mask) - 1;
-	} else if (w < r) {
-		free_cnt = (r - w) - 1;
-	} else {
-		free_cnt = rb->size - 1;
-	}
-
+	free_cnt = jack_ringbuffer_write_space(rb);
 	cnt2 = w + free_cnt;
 
 	if (cnt2 > rb->size) {