[PATCH] ipipe_tsc: Use WRTIE_ONCE for updating last_tsc

[Greg Gallagher]

On Tue, May 11, 2021 at 11:31 AM Greg Gallagher <greg at embeddedgreg.com>
wrote:

>
>
> On Tue, May 11, 2021 at 11:25 AM Jan Kiszka <jan.kiszka at siemens.com>
> wrote:
>
>> On 11.05.21 16:57, Grau, Gunter via Xenomai wrote:
>> >> -----Original Message-----
>> >> From: Gunter Grau <gunter.grau at philips.com>
>> >> Sent: Freitag, 5. März 2021 09:22
>> >> To: xenomai at xenomai.org
>> >> Cc: rpm at xenomai.org; Grau, Gunter <gunter.grau at philips.com>
>> >> Subject: [PATCH] ipipe_tsc: Use WRTIE_ONCE for updating last_tsc
>> >>
>> >> The gcc 8.3 compiler generated for the update of the last_tsc
>> >> 8 byte value two store commands (see __ipipe_update_tsc).
>> >>
>> >>         e2500001        subs    r0, r0, #1
>> >>         e5830000        str     r0, [r3]
>> >>         e2c11000        sbc     r1, r1, #0
>> >>         e5831004        str     r1, [r3, #4]
>> >>
>> >> This may lead to wrong reading of the counter in the assembler part
>> >> __ipipe_tsc_get if a CPU core just reads this value when only half of
>> the
>> >> last_tsc field is updated.
>> >>
>> >>         __ipipe_freerunning_32:
>> >>         ldr     r0, .LCfr32_cntr_addr
>> >>         /* User-space entry-point: r0 is the hardware counter virtual
>> address */
>> >>         myldrd  r2, r3, r1, .LCfr32_last_tsc
>> >>         #ifndef CONFIG_CPU_BIG_ENDIAN
>> >>         /* Little endian */
>> >>         ldr     r0, [r0]
>> >>         cmp     r2, r0
>> >>         adc     r1, r3, #0
>> >>
>> >> The issue can be seen when enabling DEBUG_TIMEKEEPING option in the
>> >> kernel and may lead to a jump backwards in time for the monotonic
>> timer in
>> >> Linux.
>> >>
>> >> WARNING: Underflow in clocksource 'ipipe_tsc' observed, time update
>> >> ignored.
>> >>          Please report this, consider using a different clocksource,
>> if possible.
>> >>          Your kernel is probably still fine.
>> >>
>> >> Fix this by using the WRITE_ONCE macro to prevent the compiler from
>> >> tearing the write since this fields are accessed from different CPU
>> cores.
>> >> After this change the resulting assembler is using a double store
>> command,
>> >> which should fix the issue.
>> >>
>> >>         e2506001        subs    r6, r0, #1
>> >>         e2c17000        sbc     r7, r1, #0
>> >>         e1c360f0        strd    r6, [r3]
>> >>
>> >> Signed-off-by: Gunter Grau <gunter.grau at philips.com>
>> >> ---
>> >>  arch/arm/kernel/ipipe_tsc.c | 2 +-
>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >>
>> >> diff --git a/arch/arm/kernel/ipipe_tsc.c b/arch/arm/kernel/ipipe_tsc.c
>> index
>> >> 7e58118254b8..6d9e1cefa964 100644
>> >> --- a/arch/arm/kernel/ipipe_tsc.c
>> >> +++ b/arch/arm/kernel/ipipe_tsc.c
>> >> @@ -199,7 +199,7 @@ void __ipipe_tsc_update(void)
>> >>
>> >>         /* Update last_tsc, in order to remain compatible with legacy
>> >>            user-space 32 bits free-running counter implementation */
>> >> -       ipipe_tsc_value->last_tsc = __ipipe_tsc_get() - 1;
>> >> +       WRITE_ONCE(ipipe_tsc_value->last_tsc, __ipipe_tsc_get() - 1);
>> >>  }
>> >>  EXPORT_SYMBOL(__ipipe_tsc_get);
>> >>
>> >> --
>> >> 2.17.1
>> >>
>> >>
>> >
>> > Hi,
>> >
>> > I wanted to ask if someone could have a look on this patch.
>> > I sent 2 patches for the ipipe at this time but did not see any follow
>> ups regarding this.
>> >
>> > The other one was just a goodie I wondered about while investigating
>> this above one.
>> >
>> > This was really a pain and I wanted to save others from having the same
>> issues.
>> > The observed issues where haning processes (looping in gettimeofday) or
>> rcu_stalls on different cores.
>> >
>> > If I see this correct this issue is present in any ipipe patch from
>> xenomai 2.6 to 3.1.  I haven't checked older ones.
>> > If the compiler decides to generate such code (which it is allowed to)
>> you will see sporadic issues of the same type. Not really reproducible.
>> >
>> > So please have a look and take into account to accept this.
>> >
>> > Thanks in advance,
>> > Gunter
>> >
>>
>> Sorry, this may have fallen through the cracks.
>>
>> Greg, did you have a look at this, or is this already in your queue?
>>
>> Jan
>>
>> --
>> Siemens AG, T RDA IOT
>> Corporate Competence Center Embedded Linux
>
>
> Sorry, this one is my queue, I’ve tested it and it looks good. I’ll apply
> it tonight, and get new patches out.
>
> Thanks
>
> Greg
>
>>
>> Applied, thanks for the patch.  Sorry it took so long, new patches will
be pushed out this weekend.

-Greg