video: treat signal like timeout as failure

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 <[email protected]>

---

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.

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 <[email protected]>

---

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.

Patch is against 3.19.0-rc5 -next-20150119

Patch was only compile-tested with exynos_defconfig

 drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c +1 -0

index 2358a2f..55a7a45 100644
@@ -157,6 +157,7 @@ int exynos_mipi_dsi_wr_data(struct mipi_dsim_device *dsim, unsigned int data_id,
 	const unsigned char *data0, unsigned int data_size)
 {
 	unsigned int check_rx_ack = 0;
+	long timeout;
 
 	if (dsim->state == DSIM_STATE_ULPS) {
 		dev_err(dsim->dev, "state is ULPS.\n");

@@ -244,9 +245,11 @@ int exynos_mipi_dsi_wr_data(struct mipi_dsim_device *dsim, unsigned int data_id,
 		exynos_mipi_dsi_wr_tx_header(dsim, data_id, data_size & 0xff,

Quoted 40 lines · Nicholas Mc Guire

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 <[email protected]>

---

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.

Patch is against 3.19.0-rc5 -next-20150119

Patch was only compile-tested with exynos_defconfig

 drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c +1 -0

index 2358a2f..55a7a45 100644
@@ -157,6 +157,7 @@ int exynos_mipi_dsi_wr_data(struct mipi_dsim_device *dsim, unsigned int data_id,
 	const unsigned char *data0, unsigned int data_size)
 {
 	unsigned int check_rx_ack = 0;
+	long timeout;
 
 	if (dsim->state == DSIM_STATE_ULPS) {
 		dev_err(dsim->dev, "state is ULPS.\n");

@@ -244,9 +245,11 @@ int exynos_mipi_dsi_wr_data(struct mipi_dsim_device *dsim, unsigned int data_id,
 		exynos_mipi_dsi_wr_tx_header(dsim, data_id, data_size & 0xff,

drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c int exynos_mipi_dsi_wr_data(struct mipi_dsim_device *dsim, unsigned int data_id, · line 252

Quoted 10 lines

drivers/video/fbdev/exynos/exynos_mipi_dsi_common.c +5 -3

 			(data_size & 0xff00) >> 8);
 
-		if (!wait_for_completion_interruptible_timeout(&dsim_wr_comp,
-							MIPI_FIFO_TIMEOUT)) {
-			dev_warn(dsim->dev, "command write timeout.\n");
+		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");

This is really silly. Let's say that the program which results in this function called is using signals (eg, alarm() with SIGALRM, or asynchronous IO with SIGIO, etc).

Why should having a SIGALRM raised print a kernel message? If this happens a lot, it will result in the kernel log being flooded with these messages.

Signals should not be seen as exceptional conditions. For some programs, they are merely asynchronous events which are a normal part of the programs operation (eg, SIGIO, SIGALRM, etc.)

Please, if you are going to handle signals, then handle them properly. If you're not going to handle them properly, don't use a wait that caters for them - use wait_for_completion_killable_timeout() which doesn't finish waiting on a signal unless the signal is going to result in the death of the program.

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 <[email protected]>

---

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.

Quoted 12 lines · Nicholas Mc Guire

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 <[email protected]>

---

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.

Quoted 6 lines

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.

Quoted 20 lines · Tomi Valkeinen

Quoted 12 lines · Nicholas Mc Guire

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 <[email protected]>

---

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.

Quoted 6 lines

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.

Quoted 7 lines

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.

Quoted 29 lines · Nicholas Mc Guire

Quoted 20 lines · Tomi Valkeinen

Quoted 12 lines · Nicholas Mc Guire

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 <[email protected]>

---

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.

Quoted 6 lines

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.

Quoted 7 lines

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.

Quoted 5 lines

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.

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*!

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.

Quoted 64 lines · Russell King - ARM Linux

Quoted 29 lines · Nicholas Mc Guire

Quoted 20 lines · Tomi Valkeinen

Quoted 12 lines · Nicholas Mc Guire

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 <[email protected]>

---

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.

Quoted 6 lines

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.

Quoted 7 lines

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.

Quoted 5 lines

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;
                }

Quoted 1 line

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 5 lines

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 14 lines

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.

Quoted 5 lines

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.

Quoted 5 lines · Russell King - ARM Linux

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.

There is drivers/gpu/drm/exynos/ which is getting a lot of updates. So...

I'd propose removing the exynos fbdev driver if the exynos drm driver offers the same functionality. I don't know if that's the case. Does the drm driver support all the devices the fbdev supports?

Also, I'm not sure if and how we can remove drivers. If exynos fbdev driver is dropped, that would perhaps break boards that have exynos fbdev in their .dts file. And if the drm driver doesn't offer the exact same /dev/fbX interface, it would break the userspace.

So I don't know if that's possible. But that's what I'd like to do, eventually, for all the fbdev drivers. Implement drm driver, remove the fbdev one.