Re: [PATCH 4/7] staging: comedi: don't allow read() on async command set up for "write"
From: Ian Abbott
Date: Thu Oct 30 2014 - 17:00:36 EST
On 30/10/14 20:45, Hartley Sweeten wrote:
On Thursday, October 30, 2014 1:28 PM, Ian Abbott wrote:
On 30/10/14 18:05, Hartley Sweeten wrote:
On Thursday, October 30, 2014 5:42 AM, Ian Abbott wrote:
[snip]
add_wait_queue(&async->wait_head, &wait);
while (nbytes > 0 && !retval) {
@@ -2249,6 +2253,10 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
retval = -EACCES;
break;
}
+ if (async->cmd.flags & CMDF_WRITE) {
+ retval = -EINVAL;
+ break;
+ }
Is this second test really needed in the while() loop?
For that matter, are the s->busy tests needed in the while() loop?
To answer your second question, some other thread using the same file
object might have cancelled the asynchronous command, causing the
current thread to see that the command is no longer active when it wakes up.
To answer your first question, that other thread might have managed to
set up another asynchronous command in before we wake up, and it might
have been set up as a "write" command (if the subdevice supports
commands in both directions). This doesn't detect the case when the
other thread has managed to set up another "read" command, but since the
current read() call hasn't read any data yet, we can just pretend we
didn't know about the original command and read data from the new
command instead. (After all, the calling thread can't prove the read()
started before the first command was cancelled, so we can just pretend
it didn't.)
But when the command is first started by do_cmd_ioctl() we have this sequence:
if (s->busy)
return -EBUSY;
...
s->busy = file;
ret = s->do_cmd(dev, s);
From then on the s->busy pointer can only be cleared in do_become_nonbusy()
(by way of a (*cancel)). So another command cannot be started until the current
command is completed.
The other thread could do its own read() after it cancelled the command,
which would clear the busy condition (once it returns 0 to indicate
end-of-file), so the current thread's read() still needs to check it.
--
-=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=-
-=( Web: http://www.mev.co.uk/ )=-
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