Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1278908

Re: ->poll() instances shouldn't be indefinitely blocking

Show all headers | View raw

On Fri, Nov 27, 2015 at 7:18 AM, Mauro Carvalho Chehab
<mchehab@osg.samsung.com> wrote:
> Al Viro <viro@ZenIV.linux.org.uk> escreveu:
>
>> Take a look at this:
>> static unsigned int gsc_m2m_poll(struct file *file,
>>                                         struct poll_table_struct *wait)
>> {
>>         struct gsc_ctx *ctx = fh_to_ctx(file->private_data);
>>         struct gsc_dev *gsc = ctx->gsc_dev;
>>         int ret;
>>
>>         if (mutex_lock_interruptible(&gsc->lock))
>>                 return -ERESTARTSYS;
>>
>>         ret = v4l2_m2m_poll(file, ctx->m2m_ctx, wait);
>>         mutex_unlock(&gsc->lock);
>>
>>         return ret;
>> }
>>
>> a) ->poll() should not return -E...; callers expect just a bitmap of
>> POLL... values.
>
> Yeah. We fixed issues like that on other drivers along the time. I guess
> this is a some bad code that people just cut-and-paste from legacy drivers
> without looking into it.

Actually, while returning -ERESTARTSYS is bogus, returning _zero_
would not be. The top-level poll() code will happily notice the
signal, and return -EINTR like poll should (unless something else is
pending, in which case it will return zero and the bits set for that
something else).

So having a driver with a ->poll() function that does that kind of
conditional locking is not wrong per se. It's just he return value
that is crap.

I also do wonder if we might not make the generic code a bit more
robust wrt things like this. The bitmask we use is only about the low
bits, so we *could* certainly allow the driver poll() functions to
return errors - possibly just ignoring them. Or perhaps have a
WARN_ON_OCNE() to find them.

Al, what do you think? The whole "generic code should be robust wrt
drivers making silly mistakes" just sounds like a good idea. Finding
these things through code inspection is all well and good, but having
a nice warning report from users might be even better.

                Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

->poll() instances shouldn't be indefinitely blocking Al Viro <viro@ZenIV.linux.org.uk> - 2015-11-27 06:10 +0100
  Re: ->poll() instances shouldn't be indefinitely blocking Mauro Carvalho Chehab <mchehab@osg.samsung.com> - 2015-11-27 16:20 +0100
    Re: ->poll() instances shouldn't be indefinitely blocking Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-27 18:50 +0100
      Re: ->poll() instances shouldn't be indefinitely blocking Al Viro <viro@ZenIV.linux.org.uk> - 2015-11-30 04:10 +0100

csiph-web