staging: most: dim2-hdm: fix race condition when closing a channel

This patch fixes race between the function poison_channel that clears
the state is_initialized and the tasklet function dim2_tasklet_fn that
checks the state is_initialized.
Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/most/hdm-dim2/dim2_hdm.c

@@ -705,12 +705,14 @@ static int poison_channel(struct most_interface *most_iface, int ch_idx)
 	if (!hdm_ch->is_initialized)
 		return -EPERM;
 
+	tasklet_disable(&dim2_tasklet);
 	spin_lock_irqsave(&dim_lock, flags);
 	hal_ret = dim_destroy_channel(&hdm_ch->ch);
 	hdm_ch->is_initialized = false;
 	if (ch_idx == dev->atx_idx)
 		dev->atx_idx = -1;
 	spin_unlock_irqrestore(&dim_lock, flags);
+	tasklet_enable(&dim2_tasklet);
 	if (hal_ret != DIM_NO_ERROR) {
 		pr_err("HAL Failed to close channel %s\n", hdm_ch->name);
 		ret = -EFAULT;