Patchwork SMTC lookup in smtc_distribute_timer

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Submitter Kevin D. Kissell
Date 2009-11-10 07:11:46
Message ID <4AF9C2EA.3090205@paralogos.com>
Download mbox | patch
Permalink /patch/621/
State Accepted
Delegated to: Ralf Baechle
Headers show

Comments

Kevin D. Kissell - 2009-11-10 07:11:46
Your failure scenario looks plausible. Mea culpa.  However, I think that
a more elegant and slightly smaller (depending on just how good
the optimizer is) fix would be:




I don't have access to SMTC-capable hardware just now, but
I guess the way to test this would be to have a test program
or kernel test stub program two events separated by the smallest
possible increment, so that the second will have passed by the
time interrupt services for the first.

          Regards,

          Kevin K.

Mikael Starvik wrote:
> Ok, my guess is something like this:
>
> 1. At the end of smtc_distribute_timer, nextstamp is valid and has already 
> passed so we goto repeat. 
> 2. Nothing updates nextstamp (only updated if the timeout is in the future 
> And we just decided it is in the past)
> 3. At the end nextstamp still has the same value so it is still valid and
> in the past.
> 4. This repeats until read_c0_count has a value which causes nextstamp to
> be in the future.
>
> One possible patch that seams to solve it for me below. This is probably 
> not the correct solution so I'll need help from the SMTC experts to review
> it and come up with the correct solution.
>
> Best Regards
> /Mikael
>
> Index: cevt-smtc.c
> ===================================================================
> RCS file: /usr/local/cvs/linux/os/linux-2.6/arch/mips/kernel/cevt-smtc.c,v
> retrieving revision 1.2
> diff -u -r1.2 cevt-smtc.c
> --- cevt-smtc.c	2 Sep 2009 10:07:51 -0000	1.2
> +++ cevt-smtc.c	10 Nov 2009 11:40:31 -0000
> @@ -223,8 +223,10 @@
>  		write_c0_compare(nextstamp);
>  		ehb();
>  		if ((nextstamp - (unsigned long)read_c0_count())
> -			> (unsigned long)LONG_MAX)
> +			> (unsigned long)LONG_MAX) {
> +				nextstamp = 0L;  
>  				goto repeat;
> +			}
>  	}
>  }
>
>
>

Patch

diff --git a/arch/mips/kernel/cevt-smtc.c b/arch/mips/kernel/cevt-smtc.c
index 98bd7de..b102e4f 100644
--- a/arch/mips/kernel/cevt-smtc.c
+++ b/arch/mips/kernel/cevt-smtc.c
@@ -173,11 +173,12 @@  void smtc_distribute_timer(int vpe)
        unsigned int mtflags;
        int cpu;
        struct clock_event_device *cd;
-       unsigned long nextstamp = 0L;
+       unsigned long nextstamp;
        unsigned long reference;
 
 
 repeat:
+       nextstamp = 0L;
        for_each_online_cpu(cpu) {
            /*
             * Find virtual CPUs within the current VPE who have