On Tue, Mar 10, 2015 at 03:39:28PM +0100, Nicholas Mc Guire wrote:

On Tue, 10 Mar 2015, Russell King - ARM Linux wrote:

quoted

On Tue, Mar 10, 2015 at 01:51:16PM +0100, Nicholas Mc Guire wrote:

quoted

On Tue, 10 Mar 2015, Tomi Valkeinen wrote:

quoted

On 20/01/15 07:23, Nicholas Mc Guire wrote:

quoted

if(!wait_for_completion_interruptible_timeout(...))
only handles the timeout case - this patch adds handling the
signal case the same as timeout and cleans up.

Signed-off-by: Nicholas Mc Guire <redacted>
---

Only the timeout case was being handled, return of 0 in 
wait_for_completion_interruptible_timeout, the signal case (-ERESTARTSYS)
was treated just like the case of successful completion, which is most 
likely not reasonable.

Note that exynos_mipi_dsi_wr_data/exynos_mipi_dsi_rd_data return values
are not checked at the call sites in s6e8ax0.c (cmd_read/cmd_write)!

This patch simply treats the signal case the same way as the timeout case,
by releasing locks and returning 0 - which might not be the right thing to
do - this needs a review by someone knowing the details of this driver.

While I agree that this patch is a bit better than the current state,
the code still looks wrong as Russell said.

I can merge this, but I'd rather have someone from Samsung look at the
code and change it to use wait_for_completion_killable_timeout() if
that's what this code is really supposed to use.

If someone that knows the details takes care of it
that is of course the best solution. If someone Samsung is 
going to look into it then it is probably best to completly
drop this speculative patch so that this does not lead
to more confusion than it does good.

IMHO, just change it to wait_for_completion_killable_timeout() - that's
a much better change than the change you're proposing.

If we think about it...  The current code uses this:

                if (!wait_for_completion_interruptible_timeout(&dsim_wr_comp,
                                                        MIPI_FIFO_TIMEOUT)) {
                        dev_warn(dsim->dev, "command write timeout.\n");
                        mutex_unlock(&dsim->lock);
                        return -EAGAIN;
                }

which has the effect of treating a signal as "success", and doesn't return
an error.  So, if the calling application receives (eg) a SIGPIPE or a
SIGALRM, we proceed as if we received the FIFO empty interrupt and doesn't
cause an error.

Your change results in:

                timeout = wait_for_completion_interruptible_timeout(
                                        &dsim_wr_comp, MIPI_FIFO_TIMEOUT);
                if (timeout <= 0) {
                        dev_warn(dsim->dev,
                                "command write timed-out/interrupted.\n");
                        mutex_unlock(&dsim->lock);
                        return -EAGAIN;
                }

which now means that this call returns -EAGAIN when a signal is raised.

but in case of wait_for_completion_killable_timeout it also would return
-ERESTARTSYS (unless I'm missreading do_wait_for_common -> signal_pending_state(state, current)) so I still think it would be better to have the
dev_warn() in the path and then when the task is killed it atleast leaves
some trace of the of what was going on ?

quoted

Now, further auditing of this exynos crap (and I really do mean crap)
shows that this function is assigned to a method called "cmd_write".
Grepping for that shows that *no caller ever checks the return value*!

yup - as was noted in the patch - and this is also why it was
not really possible to figure out what should really be done
as it runs into a dead end in all cases - the only point of the patch was
to atleast generate a debug message and return some signal
indicating error ... which is then unhandled...
 

quoted

So, really, there's a bug here in that we should _never_ complete on a
signal, and we most *definitely can not* error out on a signal either.
The *only* sane change to this code without author/maintainer input is
to change this to wait_for_completion_killable_timeout() - so that
signals do not cause either premature completion nor premature failure
of the wait.

The proper fix is absolutely huge: all call paths need to be augmented
with code to detect this function failing, and back out whatever changes
they've made, and restoring the previous state (if they can) and
propagate the error all the way back to userland, so that syscall
restarting can work correctly.  _Only then_ is it safe to use a call
which causes an interruptible sleep.

Personally, I'd be happier seeing this moved into drivers/staging and
eventually deleted from the kernel unless someone is willing to review
the driver and fix some of these glaring problems.  I wouldn't be
surprised if there was _loads_ of this kind of crap there.

there is plenty of this - actually all of the wait_for_completion* related
findings I've been posting in the past 2 month are based on the attempt to
write up a more or less complete API spec in form of coccinelle scripts that
then can be used to scan and sometimes fix-up this kind of problems - but of
course just "local-fixes" - this can't fix fundamentally broken code.

In which case, let me propose that the exynos fbdev driver needs to be
moved to drivers/staging, and stay there until this stuff gets fixed.
drivers/staging is supposed to be for stuff which isn't up to the mark,
and which is potentially unstable.  And that's what this driver exactly
is.

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