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    SubjectRe: [PATCH] sched_groups are expected to be circular linked list, make it so right after allocation
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    On Wed, 2012-05-09 at 13:58 +0200, Igor Mammedov wrote:
    > On 05/09/2012 01:52 PM, Peter Zijlstra wrote:
    > > On Wed, 2012-05-09 at 13:44 +0200, Igor Mammedov wrote:
    > >> This patch fixes only build_sched_groups path, but there is another fail path
    > >> that results in below OOPS.
    > >> build_overlap_sched_groups() may exit without setting groups and later it will crash
    > >> init_sched_groups_power as well.
    > >
    > > if that allocation fails? Or is there another fail path?
    >
    > build_overlap_sched_groups(struct sched_domain *sd, int cpu)
    > ...
    > if (cpumask_test_cpu(cpu, sg_span))
    > groups = sg;
    > ...
    >
    > above test fails and leaves local var groups set to NULL
    > and before exit there is:
    >
    > sd->groups = groups;
    >
    > which resets sd->groups to NULL

    Cute! So we're building groups for @cpu, for a domain on the same @cpu,
    but none of the groups actually span this @cpu. This would imply the
    domain doesn't actually contain @cpu.

    > and I'm not sure if it is correct at all to skip this
    > assignment if groups == NULL.

    It would avoid exploding, but nothing in the above situation is anywhere
    near correct.

    Does something like the below give any clues as to how we got there?

    ---
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1043,6 +1043,15 @@ struct sched_domain {
    unsigned long span[0];
    };

    +static inline char *sched_domain_name(struct sched_domain *sd)
    +{
    +#ifdef CONFIG_SCHED_DEBUG
    + return sd->name;
    +#else
    + return "";
    +#endif
    +}
    +
    static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
    {
    return to_cpumask(sd->span);
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -5993,6 +5993,22 @@ build_overlap_sched_groups(struct sched_
    last = sg;
    last->next = first;
    }
    + if (!groups) {
    + char str[256];
    +
    + printk(KERN_ERR "sched: Topology is hosed for CPU-%d!!\n", cpu);
    + cpulist_scnprintf(str, sizeof(str), sched_domain_span(sd));
    + printk(KERN_ERR "sched: domain: %s %s\n", sched_domain_name(sd), str);
    +
    + sg = first;
    + if (sg) do {
    + cpulist_scnprintf(str, sizeof(str), sched_group_cpus(sg));
    + printk(KERN_ERR "sched: group: %s\n", str);
    + sg = sg->next;
    + } while (sg != first);
    +
    + BUG();
    + }
    sd->groups = groups;

    return 0;


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