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Subject[PATCH 1/8] staging: comedi: rearrange comedi_write() code
Date
Rearrange the code in `comedi_write()` to reduce the amount of
indentation. The code never reiterates the `while` loop once `count`
has become non-zero, so we can check that in the `while` condition to
save an indentation level. (Note that `nbytes` has been checked to be
non-zero before entering the loop, so we can remove that check.) Move
the code that makes the subdevice "become non-busy" outside the `while`
loop, using a new flag variable `become_nonbusy` to decide whether it
needs to be done. This simplifies the wait queue handling so there is a
single place where the task is removed from the wait queue, and we can
remove the `on_wait_queue` flag variable.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
---
drivers/staging/comedi/comedi_fops.c | 71 +++++++++++++++---------------------
1 file changed, 30 insertions(+), 41 deletions(-)

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 7b4af51..c9da6f3 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2307,7 +2307,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
DECLARE_WAITQUEUE(wait, current);
struct comedi_file *cfp = file->private_data;
struct comedi_device *dev = cfp->dev;
- bool on_wait_queue = false;
+ bool become_nonbusy = false;
bool attach_locked;
unsigned int old_detach_count;

@@ -2342,48 +2342,16 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
}

add_wait_queue(&async->wait_head, &wait);
- on_wait_queue = true;
- while (nbytes > 0 && !retval) {
+ while (count == 0 && !retval) {
unsigned runflags;

set_current_state(TASK_INTERRUPTIBLE);

runflags = comedi_get_subdevice_runflags(s);
if (!comedi_is_runflags_running(runflags)) {
- if (count == 0) {
- struct comedi_subdevice *new_s;
-
- if (comedi_is_runflags_in_error(runflags))
- retval = -EPIPE;
- else
- retval = 0;
- /*
- * To avoid deadlock, cannot acquire dev->mutex
- * while dev->attach_lock is held. Need to
- * remove task from the async wait queue before
- * releasing dev->attach_lock, as it might not
- * be valid afterwards.
- */
- remove_wait_queue(&async->wait_head, &wait);
- on_wait_queue = false;
- up_read(&dev->attach_lock);
- attach_locked = false;
- mutex_lock(&dev->mutex);
- /*
- * Become non-busy unless things have changed
- * behind our back. Checking dev->detach_count
- * is unchanged ought to be sufficient (unless
- * there have been 2**32 detaches in the
- * meantime!), but check the subdevice pointer
- * as well just in case.
- */
- new_s = comedi_file_write_subdevice(file);
- if (dev->attached &&
- old_detach_count == dev->detach_count &&
- s == new_s && new_s->async == async)
- do_become_nonbusy(dev, s);
- mutex_unlock(&dev->mutex);
- }
+ if (comedi_is_runflags_in_error(runflags))
+ retval = -EPIPE;
+ become_nonbusy = true;
break;
}

@@ -2433,12 +2401,33 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
nbytes -= n;

buf += n;
- break; /* makes device work like a pipe */
}
-out:
- if (on_wait_queue)
- remove_wait_queue(&async->wait_head, &wait);
+ remove_wait_queue(&async->wait_head, &wait);
set_current_state(TASK_RUNNING);
+ if (become_nonbusy && count == 0) {
+ struct comedi_subdevice *new_s;
+
+ /*
+ * To avoid deadlock, cannot acquire dev->mutex
+ * while dev->attach_lock is held.
+ */
+ up_read(&dev->attach_lock);
+ attach_locked = false;
+ mutex_lock(&dev->mutex);
+ /*
+ * Check device hasn't become detached behind our back.
+ * Checking dev->detach_count is unchanged ought to be
+ * sufficient (unless there have been 2**32 detaches in the
+ * meantime!), but check the subdevice pointer as well just in
+ * case.
+ */
+ new_s = comedi_file_write_subdevice(file);
+ if (dev->attached && old_detach_count == dev->detach_count &&
+ s == new_s && new_s->async == async)
+ do_become_nonbusy(dev, s);
+ mutex_unlock(&dev->mutex);
+ }
+out:
if (attach_locked)
up_read(&dev->attach_lock);

--
2.6.2


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