media: dvb-core: fix wrong reinitialization of ringbuffer on reopen

dvb_dvr_open() calls dvb_ringbuffer_init() when a new reader opens the
DVR device.  dvb_ringbuffer_init() calls init_waitqueue_head(), which
reinitializes the waitqueue list head to empty.

Since dmxdev->dvr_buffer.queue is a shared waitqueue (all opens of the
same DVR device share it), this orphans any existing waitqueue entries
from io_uring poll or epoll, leaving them with stale prev/next pointers
while the list head is reset to {self, self}.

The waitqueue and spinlock in dvr_buffer are already properly
initialized once in dvb_dmxdev_init().  The open path only needs to
reset the buffer data pointer, size, and read/write positions.

Replace the dvb_ringbuffer_init() call in dvb_dvr_open() with direct
assignment of data/size and a call to dvb_ringbuffer_reset(), which
properly resets pread, pwrite, and error with correct memory ordering
without touching the waitqueue or spinlock.

Cc: [email protected]
Fixes: 34731df ("V4L/DVB (3501): Dmxdev: use dvb_ringbuffer")
Reported-by: [email protected]
Tested-by: [email protected]
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: axboe

drivers/media/dvb-core/dmxdev.c

@@ -168,7 +168,9 @@ static int dvb_dvr_open(struct inode *inode, struct file *file)
 			mutex_unlock(&dmxdev->mutex);
 			return -ENOMEM;
 		}
-		dvb_ringbuffer_init(&dmxdev->dvr_buffer, mem, DVR_BUFFER_SIZE);
+		dmxdev->dvr_buffer.data = mem;
+		dmxdev->dvr_buffer.size = DVR_BUFFER_SIZE;
+		dvb_ringbuffer_reset(&dmxdev->dvr_buffer);
 		if (dmxdev->may_do_mmap)
 			dvb_vb2_init(&dmxdev->dvr_vb2_ctx, "dvr",
 				     &dmxdev->mutex,