On Sat, Nov 21, 2020 at 09:30:00AM +0100, Takashi Iwai wrote:

On Fri, 20 Nov 2020 19:33:52 +0100, Gustavo A. R. Silva wrote:

...

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by explicitly adding a break statement instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115 Signed-off-by: Gustavo A. R. Silva gustavoars@kernel.org

Thanks, applied.

Thanks for all you've taken, Takashi. -- Gustavo

On Fri, 20 Nov 2020 19:34:01 +0100, Gustavo A. R. Silva wrote:

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by explicitly adding a break statement instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115 Signed-off-by: Gustavo A. R. Silva gustavoars@kernel.org

Thanks, applied.

Takashi

On Fri, 20 Nov 2020 19:34:12 +0100, Gustavo A. R. Silva wrote:

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by explicitly adding a break statement instead of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115 Signed-off-by: Gustavo A. R. Silva gustavoars@kernel.org

Thanks, applied.

Takashi

On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:

...

This series aims to fix almost all remaining fall-through warnings in order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly add multiple break/goto/return/fallthrough statements instead of just letting the code fall through to the next case.

Notice that in order to enable -Wimplicit-fallthrough for Clang, this change[1] is meant to be reverted at some point. So, this patch helps to move in that direction.

Something important to mention is that there is currently a discrepancy between GCC and Clang when dealing with switch fall-through to empty case statements or to cases that only contain a break/continue/return statement[2][3][4].

Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped find?

IMVHO compiler warnings are supposed to warn about issues that could be bugs. Falling through to default: break; can hardly be a bug?!

It's certainly a place where the intent is not always clear. I think this makes all the cases unambiguous, and doesn't impact the machine code, since the compiler will happily optimize away any behavioral redundancy.

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:

On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:

...

On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:

...

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:

...

This series aims to fix almost all remaining fall-through warnings in order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly add multiple break/goto/return/fallthrough statements instead of just letting the code fall through to the next case.

Notice that in order to enable -Wimplicit-fallthrough for Clang, this change[1] is meant to be reverted at some point. So, this patch helps to move in that direction.

Something important to mention is that there is currently a discrepancy between GCC and Clang when dealing with switch fall-through to empty case statements or to cases that only contain a break/continue/return statement[2][3][4].

Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped find?

IMVHO compiler warnings are supposed to warn about issues that could be bugs. Falling through to default: break; can hardly be a bug?!

It's certainly a place where the intent is not always clear. I think this makes all the cases unambiguous, and doesn't impact the machine code, since the compiler will happily optimize away any behavioral redundancy.

If none of the 140 patches here fix a real bug, and there is no change to machine code then it sounds to me like a W=2 kind of a warning.

I'd like to avoid splitting common -W options between default and W=2 just based on the compiler. Getting -Wimplicit-fallthrough enabled found plenty of bugs, so making sure it works correctly for both compilers feels justified to me. (This is just a subset of the same C language short-coming.)

I think clang is just being annoying here, but if I'm the only one who feels this way chances are I'm wrong :)

It's being pretty pedantic, but I don't think it's unreasonable to explicitly state how every case ends. GCC's silence for the case of "fall through to a break" doesn't really seem justified.

On Sun, 2020-11-22 at 08:17 -0800, Kees Cook wrote:

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:

...

On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:

...

On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:

...

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:

...

This series aims to fix almost all remaining fall-through warnings in order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly add multiple break/goto/return/fallthrough statements instead of just letting the code fall through to the next case.

Notice that in order to enable -Wimplicit-fallthrough for Clang, this change[1] is meant to be reverted at some point. So, this patch helps to move in that direction.

Something important to mention is that there is currently a discrepancy between GCC and Clang when dealing with switch fall-through to empty case statements or to cases that only contain a break/continue/return statement[2][3][4].

Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped find?

IMVHO compiler warnings are supposed to warn about issues that could be bugs. Falling through to default: break; can hardly be a bug?!

It's certainly a place where the intent is not always clear. I think this makes all the cases unambiguous, and doesn't impact the machine code, since the compiler will happily optimize away any behavioral redundancy.

If none of the 140 patches here fix a real bug, and there is no change to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far: https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=K...

Well, it's a problem in an error leg, sure, but it's not a really compelling reason for a 141 patch series, is it? All that fixing this error will do is get the driver to print "oh dear there's a problem" under four more conditions than it previously did.

We've been at this for three years now with nearly a thousand patches, firstly marking all the fall throughs with /* fall through */ and later changing it to fallthrough. At some point we do have to ask if the effort is commensurate with the protection afforded. Please tell me our reward for all this effort isn't a single missing error print.

James

On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:

On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:

...

Please tell me our reward for all this effort isn't a single missing error print.

There were quite literally dozens of logical defects found by the fallthrough additions. Very few were logging only.

So can you give us the best examples (or indeed all of them if someone is keeping score)? hopefully this isn't a US election situation ...

James

On Sun, 2020-11-22 at 11:22 -0800, Joe Perches wrote:

On Sun, 2020-11-22 at 11:12 -0800, James Bottomley wrote:

...

On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:

...

On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:

...

Please tell me our reward for all this effort isn't a single missing error print.

There were quite literally dozens of logical defects found by the fallthrough additions. Very few were logging only.

So can you give us the best examples (or indeed all of them if someone is keeping score)? hopefully this isn't a US election situation ...

Gustavo? Are you running for congress now?

https://lwn.net/Articles/794944/

That's 21 reported fixes of which about 50% seem to produce no change in code behaviour at all, a quarter seem to have no user visible effect with the remaining quarter producing unexpected errors on obscure configuration parameters, which is why no-one really noticed them before.

James

On Mon, 2020-11-23 at 07:03 -0600, Gustavo A. R. Silva wrote:

On Sun, Nov 22, 2020 at 11:53:55AM -0800, James Bottomley wrote:

...

On Sun, 2020-11-22 at 11:22 -0800, Joe Perches wrote:

...

On Sun, 2020-11-22 at 11:12 -0800, James Bottomley wrote:

...

On Sun, 2020-11-22 at 10:25 -0800, Joe Perches wrote:

...

On Sun, 2020-11-22 at 10:21 -0800, James Bottomley wrote:

...

Please tell me our reward for all this effort isn't a single missing error print.

There were quite literally dozens of logical defects found by the fallthrough additions. Very few were logging only.

So can you give us the best examples (or indeed all of them if someone is keeping score)? hopefully this isn't a US election situation ...

Gustavo? Are you running for congress now?

https://lwn.net/Articles/794944/

That's 21 reported fixes of which about 50% seem to produce no change in code behaviour at all, a quarter seem to have no user visible effect with the remaining quarter producing unexpected errors on obscure configuration parameters, which is why no-one really noticed them before.

The really important point here is the number of bugs this has prevented and will prevent in the future. See an example of this, below:

https://lore.kernel.org/linux-iio/20190813135802.GB27392@kroah.com/

I think this falls into the same category as the other six bugs: it changes the output/input for parameters but no-one has really noticed, usually because the command is obscure or the bias effect is minor.

This work is still relevant, even if the total number of issues/bugs we find in the process is zero (which is not the case).

Really, no ... something which produces no improvement has no value at all ... we really shouldn't be wasting maintainer time with it because it has a cost to merge. I'm not sure we understand where the balance lies in value vs cost to merge but I am confident in the zero value case.

"The sucky thing about doing hard work to deploy hardening is that the result is totally invisible by definition (things not happening) [..]"

• Dmitry Vyukov

Really, no. Something that can't be measured at all doesn't exist.

And actually hardening is one of those things you can measure (which I do have to admit isn't true for everything in the security space) ... it's number of exploitable bugs found before you did it vs number of exploitable bugs found after you did it. Usually hardening eliminates a class of bug, so the way I've measured hardening before is to go through the CVE list for the last couple of years for product X, find all the bugs that are of the class we're looking to eliminate and say if we had hardened X against this class of bug we'd have eliminated Y% of the exploits. It can be quite impressive if Y is a suitably big number.

James

On Sun, 2020-11-22 at 21:35 +0100, Miguel Ojeda wrote:

On Sun, Nov 22, 2020 at 7:22 PM James Bottomley James.Bottomley@hansenpartnership.com wrote:

...

Well, it's a problem in an error leg, sure, but it's not a really compelling reason for a 141 patch series, is it? All that fixing this error will do is get the driver to print "oh dear there's a problem" under four more conditions than it previously did.

We've been at this for three years now with nearly a thousand patches, firstly marking all the fall throughs with /* fall through */ and later changing it to fallthrough. At some point we do have to ask if the effort is commensurate with the protection afforded. Please tell me our reward for all this effort isn't a single missing error print.

It isn't that much effort, isn't it?

Well, it seems to be three years of someone's time plus the maintainer review time and series disruption of nearly a thousand patches. Let's be conservative and assume the producer worked about 30% on the series and it takes about 5-10 minutes per patch to review, merge and for others to rework existing series. So let's say it's cost a person year of a relatively junior engineer producing the patches and say 100h of review and application time. The latter is likely the big ticket item because it's what we have in least supply in the kernel (even though it's 20x vs the producer time).

Plus we need to take into account the future mistakes that it might prevent, too. So even if there were zero problems found so far, it is still a positive change.

Well, the question I was asking is if it's worth the cost which I've tried to outline above.

I would agree if these changes were high risk, though; but they are almost trivial.

It's not about the risk of the changes it's about the cost of implementing them. Even if you discount the producer time (which someone gets to pay for, and if I were the engineering manager, I'd be unhappy about), the review/merge/rework time is pretty significant in exchange for six minor bug fixes. Fine, when a new compiler warning comes along it's certainly reasonable to see if we can benefit from it and the fact that the compiler people think it's worthwhile is enough evidence to assume this initially. But at some point you have to ask whether that assumption is supported by the evidence we've accumulated over the time we've been using it. And if the evidence doesn't support it perhaps it is time to stop the experiment.

James

On Mon, 2020-11-23 at 15:19 +0100, Miguel Ojeda wrote:

On Sun, Nov 22, 2020 at 11:36 PM James Bottomley James.Bottomley@hansenpartnership.com wrote:

...

Well, it seems to be three years of someone's time plus the maintainer review time and series disruption of nearly a thousand patches. Let's be conservative and assume the producer worked about 30% on the series and it takes about 5-10 minutes per patch to review, merge and for others to rework existing series. So let's say it's cost a person year of a relatively junior engineer producing the patches and say 100h of review and application time. The latter is likely the big ticket item because it's what we have in least supply in the kernel (even though it's 20x vs the producer time).

How are you arriving at such numbers? It is a total of ~200 trivial lines.

Well, I used git. It says that as of today in Linus' tree we have 889 patches related to fall throughs and the first series went in in october 2017 ... ignoring a couple of outliers back to February.

...

It's not about the risk of the changes it's about the cost of implementing them. Even if you discount the producer time (which someone gets to pay for, and if I were the engineering manager, I'd be unhappy about), the review/merge/rework time is pretty significant in exchange for six minor bug fixes. Fine, when a new compiler warning comes along it's certainly reasonable to see if we can benefit from it and the fact that the compiler people think it's worthwhile is enough evidence to assume this initially. But at some point you have to ask whether that assumption is supported by the evidence we've accumulated over the time we've been using it. And if the evidence doesn't support it perhaps it is time to stop the experiment.

Maintainers routinely review 1-line trivial patches, not to mention internal API changes, etc.

We're also complaining about the inability to recruit maintainers:

https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_t...

And burn out:

http://antirez.com/news/129

The whole crux of your argument seems to be maintainers' time isn't important so we should accept all trivial patches ... I'm pushing back on that assumption in two places, firstly the valulessness of the time and secondly that all trivial patches are valuable.

If some company does not want to pay for that, that's fine, but they don't get to be maintainers and claim `Supported`.

What I'm actually trying to articulate is a way of measuring value of the patch vs cost ... it has nothing really to do with who foots the actual bill.

One thesis I'm actually starting to formulate is that this continual devaluing of maintainers is why we have so much difficulty keeping and recruiting them.

James

On Mon, 2020-11-23 at 07:58 -0800, James Bottomley wrote:

We're also complaining about the inability to recruit maintainers:

https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_t...

And burn out:

http://antirez.com/news/129

https://www.wired.com/story/open-source-coders-few-tired/

What I'm actually trying to articulate is a way of measuring value of the patch vs cost ... it has nothing really to do with who foots the actual bill.

It's unclear how to measure value in consistency.

But one way that costs can be reduced is by automation and _not_ involving maintainers when the patch itself is provably correct.

One thesis I'm actually starting to formulate is that this continual devaluing of maintainers is why we have so much difficulty keeping and recruiting them.

The linux kernel has something like 1500 different maintainers listed in the MAINTAINERS file. That's not a trivial number.

$ git grep '^M:' MAINTAINERS | sort | uniq -c | wc -l 1543 $ git grep '^M:' MAINTAINERS| cut -f1 -d'<' | sort | uniq -c | wc -l 1446

I think the question you are asking is about trust and how it effects development.

And back to that wired story, the actual number of what you might be considering to be maintainers is likely less than 10% of the listed numbers above.

On Mon, 2020-11-23 at 19:56 +0100, Miguel Ojeda wrote:

On Mon, Nov 23, 2020 at 4:58 PM James Bottomley James.Bottomley@hansenpartnership.com wrote:

...

Well, I used git. It says that as of today in Linus' tree we have 889 patches related to fall throughs and the first series went in in october 2017 ... ignoring a couple of outliers back to February.

I can see ~10k insertions over ~1k commits and 15 years that mention a fallthrough in the entire repo. That is including some commits (like the biggest one, 960 insertions) that have nothing to do with C fallthrough. A single kernel release has an order of magnitude more changes than this...

But if we do the math, for an author, at even 1 minute per line change and assuming nothing can be automated at all, it would take 1 month of work. For maintainers, a couple of trivial lines is noise compared to many other patches.

So you think a one line patch should take one minute to produce ... I really don't think that's grounded in reality. I suppose a one line patch only takes a minute to merge with b4 if no-one reviews or tests it, but that's not really desirable.

In fact, this discussion probably took more time than the time it would take to review the 200 lines. :-)

I'm framing the discussion in terms of the whole series of changes we have done for fall through, both what's in the tree currently (889 patches) both in terms of the produce and the consumer. That's what I used for my figures for cost.

...

We're also complaining about the inability to recruit maintainers:

https://www.theregister.com/2020/06/30/hard_to_find_linux_maintainers_says_t...

And burn out:

http://antirez.com/news/129

Accepting trivial and useful 1-line patches

Part of what I'm trying to measure is the "and useful" bit because that's not a given.

is not what makes a voluntary maintainer quit...

so the proverb "straw which broke the camel's back" uniquely doesn't apply to maintainers

Thankless work with demanding deadlines is.

That's another potential reason, but it doesn't may other reasons less valid.

...

The whole crux of your argument seems to be maintainers' time isn't important so we should accept all trivial patches

I have not said that, at all. In fact, I am a voluntary one and I welcome patches like this. It takes very little effort on my side to review and it helps the kernel overall.

Well, you know, subsystems are very different in terms of the amount of patches a maintainer has to process per release cycle of the kernel. If a maintainer is close to capacity, additional patches, however trivial, become a problem. If a maintainer has spare cycles, trivial patches may look easy.

Paid maintainers are the ones that can take care of big features/reviews.

...

What I'm actually trying to articulate is a way of measuring value of the patch vs cost ... it has nothing really to do with who foots the actual bill.

I understand your point, but you were the one putting it in terms of a junior FTE.

No, I evaluated the producer side in terms of an FTE. What we're mostly arguing about here is the consumer side: the maintainers and people who have to rework their patch sets. I estimated that at 100h.

In my view, 1 month-work (worst case) is very much worth removing a class of errors from a critical codebase.

...

One thesis I'm actually starting to formulate is that this continual devaluing of maintainers is why we have so much difficulty keeping and recruiting them.

That may very well be true, but I don't feel anybody has devalued maintainers in this discussion.

You seem to be saying that because you find it easy to merge trivial patches, everyone should. I'm reminded of a friend long ago who thought being a Tees River Pilot was a sinecure because he could navigate the Tees blindfold. What he forgot, of course, is that just because it's easy with a trawler doesn't mean it's easy with an oil tanker. In fact it takes longer to qualify as a Tees River Pilot than it does to get a PhD.

James

On Mon, 2020-11-23 at 09:54 +1100, Finn Thain wrote:

But is anyone keeping score of the regressions? If unreported bugs count, what about unreported regressions?

Well, I was curious about the former (obviously no tool will tell me about the latter), so I asked git what patches had a fall-through series named in a fixes tag and these three popped out:

9cf51446e686 bpf, powerpc: Fix misuse of fallthrough in bpf_jit_comp() 6a9dc5fd6170 lib: Revert use of fallthrough pseudo-keyword in lib/ 91dbd73a1739 mips/oprofile: Fix fallthrough placement

I don't think any of these is fixing a significant problem, but they did cause someone time and trouble to investigate.

James

On Mon, 23 Nov 2020, Miguel Ojeda wrote:

On Mon, 23 Nov 2020, Finn Thain wrote:

...

On Sun, 22 Nov 2020, Miguel Ojeda wrote:

...

It isn't that much effort, isn't it? Plus we need to take into account the future mistakes that it might prevent, too.

We should also take into account optimisim about future improvements in tooling.

Not sure what you mean here. There is no reliable way to guess what the intention was with a missing fallthrough, even if you parsed whitespace and indentation.

What I meant was that you've used pessimism as if it was fact.

For example, "There is no way to guess what the effect would be if the compiler trained programmers to add a knee-jerk 'break' statement to avoid a warning".

Moreover, what I meant was that preventing programmer mistakes is a problem to be solved by development tools. The idea that retro-fitting new language constructs onto mature code is somehow necessary to "prevent future mistakes" is entirely questionable.

...

...

So even if there were zero problems found so far, it is still a positive change.

It is if you want to spin it that way.

How is that a "spin"? It is a fact that we won't get *implicit* fallthrough mistakes anymore (in particular if we make it a hard error).

Perhaps "handwaving" is a better term?

...

...

I would agree if these changes were high risk, though; but they are almost trivial.

This is trivial:

case 1: this();

•   fallthrough;
  

case 2: that();

But what we inevitably get is changes like this:

case 3: this();

•   break;
  

case 4: hmmm();

Why? Mainly to silence the compiler. Also because the patch author argued successfully that they had found a theoretical bug, often in mature code.

If someone changes control flow, that is on them. Every kernel developer knows what `break` does.

Sure. And if you put -Wimplicit-fallthrough into the Makefile and if that leads to well-intentioned patches that cause regressions, it is partly on you.

Have you ever considered the overall cost of the countless -Wpresume-incompetence flags?

Perhaps you pay the power bill for a build farm that produces logs that no-one reads? Perhaps you've run git bisect, knowing that the compiler messages are not interesting? Or compiled software in using a language that generates impenetrable messages? If so, here's a tip:

# grep CFLAGS /etc/portage/make.conf CFLAGS="... -Wno-all -Wno-extra ..." CXXFLAGS="${CFLAGS}"

Now allow me some pessimism: the hardware upgrades, gigawatt hours and wait time attributable to obligatory static analyses are a net loss.

...

But is anyone keeping score of the regressions? If unreported bugs count, what about unreported regressions?

Introducing `fallthrough` does not change semantics. If you are really keen, you can always compare the objects because the generated code shouldn't change.

No, it's not for me to prove that such patches don't affect code generation. That's for the patch author and (unfortunately) for reviewers.

Cheers, Miguel

On Mon, 23 Nov 2020, Joe Perches wrote:

On Tue, 2020-11-24 at 11:58 +1100, Finn Thain wrote:

...

it's not for me to prove that such patches don't affect code generation. That's for the patch author and (unfortunately) for reviewers.

Ideally, that proof would be provided by the compilation system itself and not patch authors nor reviewers nor maintainers.

Unfortunately gcc does not guarantee repeatability or deterministic output. To my knowledge, neither does clang.

Yes, I've said the same thing myself. But having attempted it, I now think this is a hard problem. YMMV.

https://lore.kernel.org/linux-scsi/alpine.LNX.2.22.394.2004281017310.12@nipp... https://lore.kernel.org/linux-scsi/alpine.LNX.2.22.394.2005211358460.8@nippy...

On Sun, Nov 22, 2020 at 8:17 AM Kees Cook keescook@chromium.org wrote:

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:

...

If none of the 140 patches here fix a real bug, and there is no change to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far: https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=K...

So looks like the bulk of these are: switch (x) { case 0: ++x; default: break; }

I have a patch that fixes those up for clang: https://reviews.llvm.org/D91895

There's 3 other cases that don't quite match between GCC and Clang I observe in the kernel: switch (x) { case 0: ++x; default: goto y; } y:;

switch (x) { case 0: ++x; default: return; }

switch (x) { case 0: ++x; default: ; }

Based on your link, and Nathan's comment on my patch, maybe Clang should continue to warn for the above (at least the `default: return;` case) and GCC should change? While the last case looks harmless, there were only 1 or 2 across the tree in my limited configuration testing; I really think we should just add `break`s for those.

On Mon, Nov 23, 2020 at 05:32:51PM -0800, Nick Desaulniers wrote:

On Sun, Nov 22, 2020 at 8:17 AM Kees Cook keescook@chromium.org wrote:

...

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:

...

If none of the 140 patches here fix a real bug, and there is no change to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far: https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=K...

So looks like the bulk of these are: switch (x) { case 0: ++x; default: break; }

This should not generate a warning.

I have a patch that fixes those up for clang: https://reviews.llvm.org/D91895

There's 3 other cases that don't quite match between GCC and Clang I observe in the kernel: switch (x) { case 0: ++x; default: goto y; } y:;

This should generate a warning.

switch (x) { case 0: ++x; default: return; }

Warn for this.

switch (x) { case 0: ++x; default: ; }

Don't warn for this.

If adding a break statement changes the flow of the code then warn about potentially missing break statements, but if it doesn't change anything then don't warn about it.

regards, dan carpenter

On Mon, Nov 23, 2020 at 04:03:45PM -0400, Jason Gunthorpe wrote:

On Fri, Nov 20, 2020 at 12:21:39PM -0600, Gustavo A. R. Silva wrote:

...

IB/hfi1: Fix fall-through warnings for Clang IB/mlx4: Fix fall-through warnings for Clang IB/qedr: Fix fall-through warnings for Clang RDMA/mlx5: Fix fall-through warnings for Clang

I picked these four to the rdma tree, thanks

Awesome. :)

Thank you, Jason. -- Gustavo

On Mon, Nov 23, 2020 at 08:38:46PM +0000, Mark Brown wrote:

On Fri, 20 Nov 2020 12:21:39 -0600, Gustavo A. R. Silva wrote:

...

This series aims to fix almost all remaining fall-through warnings in order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly add multiple break/goto/return/fallthrough statements instead of just letting the code fall through to the next case.

[...]

Applied to

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator.git for-next

Thanks!

[1/1] regulator: as3722: Fix fall-through warnings for Clang commit: b52b417ccac4fae5b1f2ec4f1d46eb91e4493dc5

Thank you, Mark. -- Gustavo