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[Xen-devel] [PATCH 1/8] cpupools: fix state when downing a CPU failed

While I've run into the issue with further patches in place which no
longer guarantee the per-CPU area to start out as all zeros, the
CPU_DOWN_FAILED processing looks to have the same issue: By not zapping
the per-CPU cpupool pointer, cpupool_cpu_add()'s (indirect) invocation
of schedule_cpu_switch() will trigger the "c != old_pool" assertion

Clearing the field during CPU_DOWN_PREPARE is too early (afaict this
should not happen before cpu_disable_scheduler()). Clearing it in
CPU_DEAD and CPU_DOWN_FAILED would be an option, but would take the same
piece of code twice. Since the field's value shouldn't matter while the
CPU is offline, simply clear it in CPU_ONLINE and CPU_DOWN_FAILED, but
only for other than the suspend/resume case (which gets specially
handled in cpupool_cpu_remove()).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
TBD: I think this would better call schedule_cpu_switch(cpu, NULL) from
     cpupool_cpu_remove(), but besides that - as per above - likely
     being too early, that function has further prereqs to be met. It
     also doesn't look as if cpupool_unassign_cpu_helper() could be used

--- a/xen/common/cpupool.c
+++ b/xen/common/cpupool.c
@@ -778,6 +778,8 @@ static int cpu_callback(
     case CPU_DOWN_FAILED:
     case CPU_ONLINE:
+        if ( system_state <= SYS_STATE_active )
+            per_cpu(cpupool, cpu) = NULL;
         rc = cpupool_cpu_add(cpu);

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