From: Michael Ellerman <mpe@ellerman.id.au> Date: 2018-09-20 09:41:14
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(+)
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 <redacted>
Just curious to know, how did you find this. Static code checker compiled
or any specific compiler warnings or just by code read?
Maddy
On Thu, Sep 20, 2018 at 03:29:22PM +0530, Madhavan Srinivasan wrote:
On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote:
quoted
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
From: Michael Ellerman <mpe@ellerman.id.au> Date: 2018-09-24 07:03:22
Segher Boessenkool [off-list ref] writes:
On Thu, Sep 20, 2018 at 03:29:22PM +0530, Madhavan Srinivasan wrote:
quoted
On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote:
quoted
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.
quoted
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:
quoted
On Thursday 20 September 2018 03:11 PM, Michael Ellerman wrote:
quoted
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).
From: Michael Ellerman <hidden> Date: 2018-10-04 06:32:20
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 <redacted>