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Subject[PATCH 3.16 054/346] [media] media: dvb_ringbuffer: Add memory barriers
3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Soeren Moch <smoch@web.de>

commit ca6e6126db5494f18c6c6615060d4d803b528bff upstream.

Implement memory barriers according to Documentation/circular-buffers.txt:
- use smp_store_release() to update ringbuffer read/write pointers
- use smp_load_acquire() to load write pointer on reader side
- use ACCESS_ONCE() to load read pointer on writer side

This fixes data stream corruptions observed e.g. on an ARM Cortex-A9
quad core system with different types (PCI, USB) of DVB tuners.

Signed-off-by: Soeren Moch <smoch@web.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/media/dvb-core/dvb_ringbuffer.c | 74 +++++++++++++++++++++++++++------
1 file changed, 61 insertions(+), 13 deletions(-)

--- a/drivers/media/dvb-core/dvb_ringbuffer.c
+++ b/drivers/media/dvb-core/dvb_ringbuffer.c
@@ -55,7 +55,13 @@ void dvb_ringbuffer_init(struct dvb_ring

int dvb_ringbuffer_empty(struct dvb_ringbuffer *rbuf)
{
- return (rbuf->pread==rbuf->pwrite);
+ /* smp_load_acquire() to load write pointer on reader side
+ * this pairs with smp_store_release() in dvb_ringbuffer_write(),
+ * dvb_ringbuffer_write_user(), or dvb_ringbuffer_reset()
+ *
+ * for memory barriers also see Documentation/circular-buffers.txt
+ */
+ return (rbuf->pread == smp_load_acquire(&rbuf->pwrite));
}


@@ -64,7 +70,12 @@ ssize_t dvb_ringbuffer_free(struct dvb_r
{
ssize_t free;

- free = rbuf->pread - rbuf->pwrite;
+ /* ACCESS_ONCE() to load read pointer on writer side
+ * this pairs with smp_store_release() in dvb_ringbuffer_read(),
+ * dvb_ringbuffer_read_user(), dvb_ringbuffer_flush(),
+ * or dvb_ringbuffer_reset()
+ */
+ free = ACCESS_ONCE(rbuf->pread) - rbuf->pwrite;
if (free <= 0)
free += rbuf->size;
return free-1;
@@ -76,7 +87,11 @@ ssize_t dvb_ringbuffer_avail(struct dvb_
{
ssize_t avail;

- avail = rbuf->pwrite - rbuf->pread;
+ /* smp_load_acquire() to load write pointer on reader side
+ * this pairs with smp_store_release() in dvb_ringbuffer_write(),
+ * dvb_ringbuffer_write_user(), or dvb_ringbuffer_reset()
+ */
+ avail = smp_load_acquire(&rbuf->pwrite) - rbuf->pread;
if (avail < 0)
avail += rbuf->size;
return avail;
@@ -86,14 +101,25 @@ ssize_t dvb_ringbuffer_avail(struct dvb_

void dvb_ringbuffer_flush(struct dvb_ringbuffer *rbuf)
{
- rbuf->pread = rbuf->pwrite;
+ /* dvb_ringbuffer_flush() counts as read operation
+ * smp_load_acquire() to load write pointer
+ * smp_store_release() to update read pointer, this ensures that the
+ * correct pointer is visible for subsequent dvb_ringbuffer_free()
+ * calls on other cpu cores
+ */
+ smp_store_release(&rbuf->pread, smp_load_acquire(&rbuf->pwrite));
rbuf->error = 0;
}
EXPORT_SYMBOL(dvb_ringbuffer_flush);

void dvb_ringbuffer_reset(struct dvb_ringbuffer *rbuf)
{
- rbuf->pread = rbuf->pwrite = 0;
+ /* dvb_ringbuffer_reset() counts as read and write operation
+ * smp_store_release() to update read pointer
+ */
+ smp_store_release(&rbuf->pread, 0);
+ /* smp_store_release() to update write pointer */
+ smp_store_release(&rbuf->pwrite, 0);
rbuf->error = 0;
}

@@ -119,12 +145,17 @@ ssize_t dvb_ringbuffer_read_user(struct
return -EFAULT;
buf += split;
todo -= split;
- rbuf->pread = 0;
+ /* smp_store_release() for read pointer update to ensure
+ * that buf is not overwritten until read is complete,
+ * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+ */
+ smp_store_release(&rbuf->pread, 0);
}
if (copy_to_user(buf, rbuf->data+rbuf->pread, todo))
return -EFAULT;

- rbuf->pread = (rbuf->pread + todo) % rbuf->size;
+ /* smp_store_release() to update read pointer, see above */
+ smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);

return len;
}
@@ -139,11 +170,16 @@ void dvb_ringbuffer_read(struct dvb_ring
memcpy(buf, rbuf->data+rbuf->pread, split);
buf += split;
todo -= split;
- rbuf->pread = 0;
+ /* smp_store_release() for read pointer update to ensure
+ * that buf is not overwritten until read is complete,
+ * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+ */
+ smp_store_release(&rbuf->pread, 0);
}
memcpy(buf, rbuf->data+rbuf->pread, todo);

- rbuf->pread = (rbuf->pread + todo) % rbuf->size;
+ /* smp_store_release() to update read pointer, see above */
+ smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);
}


@@ -158,10 +194,16 @@ ssize_t dvb_ringbuffer_write(struct dvb_
memcpy(rbuf->data+rbuf->pwrite, buf, split);
buf += split;
todo -= split;
- rbuf->pwrite = 0;
+ /* smp_store_release() for write pointer update to ensure that
+ * written data is visible on other cpu cores before the pointer
+ * update, this pairs with smp_load_acquire() in
+ * dvb_ringbuffer_empty() or dvb_ringbuffer_avail()
+ */
+ smp_store_release(&rbuf->pwrite, 0);
}
memcpy(rbuf->data+rbuf->pwrite, buf, todo);
- rbuf->pwrite = (rbuf->pwrite + todo) % rbuf->size;
+ /* smp_store_release() for write pointer update, see above */
+ smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);

return len;
}
@@ -181,12 +223,18 @@ ssize_t dvb_ringbuffer_write_user(struct
return len - todo;
buf += split;
todo -= split;
- rbuf->pwrite = 0;
+ /* smp_store_release() for write pointer update to ensure that
+ * written data is visible on other cpu cores before the pointer
+ * update, this pairs with smp_load_acquire() in
+ * dvb_ringbuffer_empty() or dvb_ringbuffer_avail()
+ */
+ smp_store_release(&rbuf->pwrite, 0);
}
status = copy_from_user(rbuf->data+rbuf->pwrite, buf, todo);
if (status)
return len - todo;
- rbuf->pwrite = (rbuf->pwrite + todo) % rbuf->size;
+ /* smp_store_release() for write pointer update, see above */
+ smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);

return len;
}
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