Re: [LAD] Atomic Operations

On 12/17/2009 04:00 PM, Tim Goetze wrote:
> [Olivier Guilyardi]
>
>> On 12/17/2009 01:03 PM, Tim Blechmann wrote:
>>>> +#if defined(__APPLE__)
>>>> +#include <libkern/OSAtomic.h>
>>>> +#define MEMORY_BARRIER() OSMemoryBarrier()
>>>> +#elif (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1)
>>>> +#define MEMORY_BARRIER() __sync_synchronize()
>>>> +#else
>>>> +#warning SMP Danger: memory barriers are not supported on this system
>>>> +#endif
>
> I may be wrong but if neither __APPLE__ nor __GNUC__ >= 4.1 are true
> shouldn't MEMORY_BARRIER() at least be an empty define to keep cpp
> happy?
>
> After all, your #warning statement seems to indicate you don't want
> compilation to fail.

Yes indeed. Fixed.

--
  Olivier

Index: libjack/ringbuffer.c
===================================================================
--- libjack/ringbuffer.c (revision 3862)
+++ libjack/ringbuffer.c (working copy)
@@ -29,6 +29,16 @@
 #endif /* USE_MLOCK */
 #include <jack/ringbuffer.h>
 
+#if defined(__APPLE__)
+#include <libkern/OSAtomic.h>
+#define MEMORY_BARRIER() OSMemoryBarrier()
+#elif (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1)
+#define MEMORY_BARRIER() __sync_synchronize()
+#else
+#warning SMP Danger: memory barriers are not supported on this system
+#define MEMORY_BARRIER()
+#endif
+
 /* Create a new ringbuffer to hold at least `sz' bytes of data. The
    actual buffer size is rounded up to the next power of two. */
 
@@ -146,6 +156,7 @@
         size_t cnt2;
         size_t to_read;
         size_t n1, n2;
+ size_t new_ptr;
 
         if ((free_cnt = jack_ringbuffer_read_space (rb)) == 0) {
                 return 0;
@@ -164,13 +175,17 @@
         }
 
         memcpy (dest, &(rb->buf[rb->read_ptr]), n1);
- rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask;
+ new_ptr = (rb->read_ptr + n1) & rb->size_mask;
 
         if (n2) {
- memcpy (dest + n1, &(rb->buf[rb->read_ptr]), n2);
- rb->read_ptr = (rb->read_ptr + n2) & rb->size_mask;
+ memcpy (dest + n1, &(rb->buf[new_ptr]), n2);
+ new_ptr = (new_ptr + n2) & rb->size_mask;
         }
 
+ /* Ensure that the read pointer gets updated after copying the data */
+ MEMORY_BARRIER();
+ rb->read_ptr = new_ptr;
+
         return to_read;
 }
 
@@ -226,6 +241,7 @@
         size_t cnt2;
         size_t to_write;
         size_t n1, n2;
+ size_t new_ptr;
 
         if ((free_cnt = jack_ringbuffer_write_space (rb)) == 0) {
                 return 0;
@@ -244,13 +260,17 @@
         }
 
         memcpy (&(rb->buf[rb->write_ptr]), src, n1);
- rb->write_ptr = (rb->write_ptr + n1) & rb->size_mask;
+ new_ptr = (rb->write_ptr + n1) & rb->size_mask;
 
         if (n2) {
- memcpy (&(rb->buf[rb->write_ptr]), src + n1, n2);
- rb->write_ptr = (rb->write_ptr + n2) & rb->size_mask;
+ memcpy (&(rb->buf[new_ptr]), src + n1, n2);
+ new_ptr = (new_ptr + n2) & rb->size_mask;
         }
 
+ /* Ensure that the write pointer gets updated after copying the data */
+ MEMORY_BARRIER();
+ rb->write_ptr = new_ptr;
+
         return to_write;
 }
 
@@ -260,6 +280,8 @@
 jack_ringbuffer_read_advance (jack_ringbuffer_t * rb, size_t cnt)
 {
         size_t tmp = (rb->read_ptr + cnt) & rb->size_mask;
+ /* Ensure that the read pointer gets updated after external data change */
+ MEMORY_BARRIER();
         rb->read_ptr = tmp;
 }
 
@@ -269,6 +291,8 @@
 jack_ringbuffer_write_advance (jack_ringbuffer_t * rb, size_t cnt)
 {
         size_t tmp = (rb->write_ptr + cnt) & rb->size_mask;
+ /* Ensure that the write pointer gets updated after external data change */
+ MEMORY_BARRIER();
         rb->write_ptr = tmp;
 }
 

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Received on Thu Dec 17 20:15:03 2009