| Message ID | 20180920094111.4125-1-mpe@ellerman.id.au (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | db6711b7a17f03921e734e11e3a1e9bccb28bf46 |
| Headers | show |
| Series | powerpc/perf: Add missing break in power7_marked_instr_event() | expand |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/apply_patch | success | next/apply_patch Successfully applied |
| snowpatch_ozlabs/checkpatch | success | Test checkpatch on branch next |
| snowpatch_ozlabs/build-ppc64le | success | Test build-ppc64le on branch next |
| snowpatch_ozlabs/build-ppc64be | success | Test build-ppc64be on branch next |
| snowpatch_ozlabs/build-ppc64e | success | Test build-ppc64e on branch next |
| snowpatch_ozlabs/build-ppc32 | success | Test build-ppc32 on branch next |
On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote: > In power7_marked_instr_event() there is a switch case that is missing > a break or an explicit fallthrough, it's not immediately clear which > it should be. > > The function determines based on the PMU event code, whether the event > is a "marked" event (which then requires us to configure the PMU in a > certain way). On Power7 there is no specific bit(s) in the event to > tell us that, we just have to know. > > Rather than having a full list of every event and whether they are > marked, we pull apart the event code and for events with certain > values of certain fields we can say that those are all marked events. > > We take the psel (bits 0-7) of the event, and look at bits 4-7. For a > value of 6 we say that if the entire psel == 0x64 then if the pmc == 3 > the event is marked, else not, and otherwise we continue. > > It is then that we fallthrough to the 8 case, where we return true if > the unit == 0xd. > > The question is should the 6 case also fallthrough and check for > unit == 0xd, or should it return. > > Looking at the full list of events we see that there are zero events > where (psel >> 4) == 0x6 and unit == 0xd. > > So the answer is it doesn't really matter, there are no valid event > codes that will return a different result whether we fallthrough or > break. > > But equally, testing the 6 case events against unit == 0xd is slightly > bogus, as there are no such events. So to make the code clearer, and > avoid any future confusion, have the 6 case break rather than falling > through. Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com> Just curious to know, how did you find this. Static code checker compiled or any specific compiler warnings or just by code read? Maddy > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> > --- > arch/powerpc/perf/power7-pmu.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/powerpc/perf/power7-pmu.c b/arch/powerpc/perf/power7-pmu.c > index 7963658dbc22..6dbae9884ec4 100644 > --- a/arch/powerpc/perf/power7-pmu.c > +++ b/arch/powerpc/perf/power7-pmu.c > @@ -238,6 +238,7 @@ static int power7_marked_instr_event(u64 event) > case 6: > if (psel == 0x64) > return pmc >= 3; > + break; > case 8: > return unit == 0xd; > }
On Thu, Sep 20, 2018 at 03:29:22PM +0530, Madhavan Srinivasan wrote: > On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote: > >In power7_marked_instr_event() there is a switch case that is missing > >a break or an explicit fallthrough, it's not immediately clear which > >it should be. > Just curious to know, how did you find this. Static code checker compiled > or any specific compiler warnings or just by code read? Newer GCC warns about suspicious fallthroughs (-Wimplicit-fallthrough, which is in -Wextra). Segher
Segher Boessenkool <segher@kernel.crashing.org> writes: > On Thu, Sep 20, 2018 at 03:29:22PM +0530, Madhavan Srinivasan wrote: >> On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote: >> >In power7_marked_instr_event() there is a switch case that is missing >> >a break or an explicit fallthrough, it's not immediately clear which >> >it should be. > >> Just curious to know, how did you find this. Static code checker compiled >> or any specific compiler warnings or just by code read? > > Newer GCC warns about suspicious fallthroughs (-Wimplicit-fallthrough, > which is in -Wextra). Yeah I actually turned -Wimplicit-fallthrough on manually. I'm hoping to add that to our CFLAGS once I've merged this fix and worked out the Kbuild magic to add it to CFLAGS just for arch/powerpc. cheers
On Thursday 20 September 2018 08:17 PM, Segher Boessenkool wrote: > On Thu, Sep 20, 2018 at 03:29:22PM +0530, Madhavan Srinivasan wrote: >> On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote: >>> In power7_marked_instr_event() there is a switch case that is missing >>> a break or an explicit fallthrough, it's not immediately clear which >>> it should be. >> Just curious to know, how did you find this. Static code checker compiled >> or any specific compiler warnings or just by code read? > Newer GCC warns about suspicious fallthroughs (-Wimplicit-fallthrough, > which is in -Wextra). > Nice good to know. Maddy > Segher >
On Thu, 2018-09-20 at 09:41:11 UTC, Michael Ellerman wrote: > In power7_marked_instr_event() there is a switch case that is missing > a break or an explicit fallthrough, it's not immediately clear which > it should be. > > The function determines based on the PMU event code, whether the event > is a "marked" event (which then requires us to configure the PMU in a > certain way). On Power7 there is no specific bit(s) in the event to > tell us that, we just have to know. > > Rather than having a full list of every event and whether they are > marked, we pull apart the event code and for events with certain > values of certain fields we can say that those are all marked events. > > We take the psel (bits 0-7) of the event, and look at bits 4-7. For a > value of 6 we say that if the entire psel == 0x64 then if the pmc == 3 > the event is marked, else not, and otherwise we continue. > > It is then that we fallthrough to the 8 case, where we return true if > the unit == 0xd. > > The question is should the 6 case also fallthrough and check for > unit == 0xd, or should it return. > > Looking at the full list of events we see that there are zero events > where (psel >> 4) == 0x6 and unit == 0xd. > > So the answer is it doesn't really matter, there are no valid event > codes that will return a different result whether we fallthrough or > break. > > But equally, testing the 6 case events against unit == 0xd is slightly > bogus, as there are no such events. So to make the code clearer, and > avoid any future confusion, have the 6 case break rather than falling > through. > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> > Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com> Applied to powerpc next. https://git.kernel.org/powerpc/c/db6711b7a17f03921e734e11e3a1e9 cheers
diff --git a/arch/powerpc/perf/power7-pmu.c b/arch/powerpc/perf/power7-pmu.c index 7963658dbc22..6dbae9884ec4 100644 --- a/arch/powerpc/perf/power7-pmu.c +++ b/arch/powerpc/perf/power7-pmu.c @@ -238,6 +238,7 @@ static int power7_marked_instr_event(u64 event) case 6: if (psel == 0x64) return pmc >= 3; + break; case 8: return unit == 0xd; }
In power7_marked_instr_event() there is a switch case that is missing a break or an explicit fallthrough, it's not immediately clear which it should be. The function determines based on the PMU event code, whether the event is a "marked" event (which then requires us to configure the PMU in a certain way). On Power7 there is no specific bit(s) in the event to tell us that, we just have to know. Rather than having a full list of every event and whether they are marked, we pull apart the event code and for events with certain values of certain fields we can say that those are all marked events. We take the psel (bits 0-7) of the event, and look at bits 4-7. For a value of 6 we say that if the entire psel == 0x64 then if the pmc == 3 the event is marked, else not, and otherwise we continue. It is then that we fallthrough to the 8 case, where we return true if the unit == 0xd. The question is should the 6 case also fallthrough and check for unit == 0xd, or should it return. Looking at the full list of events we see that there are zero events where (psel >> 4) == 0x6 and unit == 0xd. So the answer is it doesn't really matter, there are no valid event codes that will return a different result whether we fallthrough or break. But equally, testing the 6 case events against unit == 0xd is slightly bogus, as there are no such events. So to make the code clearer, and avoid any future confusion, have the 6 case break rather than falling through. Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> --- arch/powerpc/perf/power7-pmu.c | 1 + 1 file changed, 1 insertion(+)