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Date: Thu, 11 Apr 2024 13:26:52 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Adrian Hunter" <adrian.hunter@intel.com>,
 "Christophe Leroy" <christophe.leroy@csgroup.eu>
Cc: "Peter Zijlstra" <peterz@infradead.org>,
 "Dave Hansen" <dave.hansen@linux.intel.com>,
 "John Stultz" <jstultz@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
 "Alexander Gordeev" <agordeev@linux.ibm.com>,
 "Vincenzo Frascino" <vincenzo.frascino@arm.com>,
 "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
 "Naresh Kamboju" <naresh.kamboju@linaro.org>,
 "x86@kernel.org" <x86@kernel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Ingo Molnar" <mingo@redhat.com>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 "Christian Borntraeger" <borntraeger@linux.ibm.com>,
 "Vasily Gorbik" <gor@linux.ibm.com>,
 "Heiko Carstens" <hca@linux.ibm.com>,
 "Nicholas Piggin" <npiggin@gmail.com>, "Borislav Petkov" <bp@alien8.de>,
 "Andy Lutomirski" <luto@kernel.org>,
 "Bjorn Helgaas" <bhelgaas@google.com>,
 "Thomas Gleixner" <tglx@linutronix.de>,
 "Anna-Maria Gleixner" <anna-maria@linutronix.de>,
 "Stephen Boyd" <sboyd@kernel.org>,
 "Randy Dunlap" <rdunlap@infradead.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG
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On Thu, Apr 11, 2024, at 11:27, Adrian Hunter wrote:
> On 11/04/24 11:22, Christophe Leroy wrote:
>> Le 11/04/2024 =C3=A0 10:12, Christophe Leroy a =C3=A9crit=C2=A0:
>>>
>>> Looking at the report, I think the correct fix should be to use=20
>>> BUILD_BUG() instead of BUG()
>>=20
>> I confirm the error goes away with the following change to next-20240=
411=20
>> on powerpc tinyconfig with gcc 13.2
>>=20
>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>> index 4e18db1819f8..3d5ac0cdd721 100644
>> --- a/kernel/time/timekeeping.c
>> +++ b/kernel/time/timekeeping.c
>> @@ -282,7 +282,7 @@ static inline void timekeeping_check_update(struc=
t=20
>> timekeeper *tk, u64 offset)
>>   }
>>   static inline u64 timekeeping_debug_get_ns(const struct tk_read_bas=
e *tkr)
>>   {
>> -	BUG();
>> +	BUILD_BUG();
>>   }
>>   #endif
>>=20
>
> That is fragile because it depends on defined(__OPTIMIZE__),
> so it should still be:

If there is a function that is defined but that must never be
called, I think we are doing something wrong. Before
e8e9d21a5df6 ("timekeeping: Refactor timekeeping helpers"),
the #ifdef made some sense, but now the #else is not really
that useful.

Ideally we would make timekeeping_debug_get_delta() and
timekeeping_check_update() just return in case of
!IS_ENABLED(CONFIG_DEBUG_TIMEKEEPING), but unfortunately
the code uses some struct members that are undefined then.

The patch below moves the #ifdef check into these functions,
which is not great, but it avoids defining useless
functions. Maybe there is a better way here. How about
just removing the BUG()?

     Arnd

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4e18db1819f8..16c6dba64dd6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -195,12 +195,11 @@ static inline u64 tk_clock_read(const struct tk_re=
ad_base *tkr)
        return clock->read(clock);
 }
=20
-#ifdef CONFIG_DEBUG_TIMEKEEPING
 #define WARNING_FREQ (HZ*300) /* 5 minute rate-limiting */
=20
 static void timekeeping_check_update(struct timekeeper *tk, u64 offset)
 {
-
+#ifdef CONFIG_DEBUG_TIMEKEEPING
        u64 max_cycles =3D tk->tkr_mono.clock->max_cycles;
        const char *name =3D tk->tkr_mono.clock->name;
=20
@@ -235,12 +234,19 @@ static void timekeeping_check_update(struct timeke=
eper *tk, u64 offset)
                }
                tk->overflow_seen =3D 0;
        }
+#endif
 }
=20
 static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *t=
kr, u64 cycles);
=20
-static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *t=
kr)
+static u64 __timekeeping_get_ns(const struct tk_read_base *tkr)
+{
+       return timekeeping_cycles_to_ns(tkr, tk_clock_read(tkr));
+}
+
+static inline u64 timekeeping_get_ns(const struct tk_read_base *tkr)
 {
+#ifdef CONFIG_DEBUG_TIMEKEEPING
        struct timekeeper *tk =3D &tk_core.timekeeper;
        u64 now, last, mask, max, delta;
        unsigned int seq;
@@ -275,16 +281,10 @@ static inline u64 timekeeping_debug_get_ns(const s=
truct tk_read_base *tkr)
=20
        /* timekeeping_cycles_to_ns() handles both under and overflow */
        return timekeeping_cycles_to_ns(tkr, now);
-}
 #else
-static inline void timekeeping_check_update(struct timekeeper *tk, u64 =
offset)
-{
-}
-static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *t=
kr)
-{
-       BUG();
-}
+       return __timekeeping_get_ns(tkr);
 #endif
+}
=20
 /**
  * tk_setup_internals - Set up internals to use clocksource clock.
@@ -390,19 +390,6 @@ static inline u64 timekeeping_cycles_to_ns(const st=
ruct tk_read_base *tkr, u64 c
        return ((delta * tkr->mult) + tkr->xtime_nsec) >> tkr->shift;
 }
=20
-static __always_inline u64 __timekeeping_get_ns(const struct tk_read_ba=
se *tkr)
-{
-       return timekeeping_cycles_to_ns(tkr, tk_clock_read(tkr));
-}
-
-static inline u64 timekeeping_get_ns(const struct tk_read_base *tkr)
-{
-       if (IS_ENABLED(CONFIG_DEBUG_TIMEKEEPING))
-               return timekeeping_debug_get_ns(tkr);
-
-       return __timekeeping_get_ns(tkr);
-}
-
 /**
  * update_fast_timekeeper - Update the fast and NMI safe monotonic time=
keeper.
  * @tkr: Timekeeping readout base from which we take the update