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Subject[ 60/72] ARM: 7767/1: let the ASID allocator handle suspended animation
Date
3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Marc Zyngier <Marc.Zyngier@arm.com>

commit ae120d9edfe96628f03d87634acda0bfa7110632 upstream.

When a CPU is running a process, the ASID for that process is
held in a per-CPU variable (the "active ASIDs" array). When
the ASID allocator handles a rollover, it copies the active
ASIDs into a "reserved ASIDs" array to ensure that a process
currently running on another CPU will continue to run unaffected.
The active array is zero-ed to indicate that a rollover occurred.

Because of this mechanism, a reserved ASID is only remembered for
a single rollover. A subsequent rollover will completely refill
the reserved ASIDs array.

In a severely oversubscribed environment where a CPU can be
prevented from running for extended periods of time (think virtual
machines), the above has a horrible side effect:

[P{a} denotes process P running with ASID a]

CPU-0 CPU-1

A{x} [active = <x 0>]

[suspended] runs B{y} [active = <x y>]

[rollover:
active = <0 0>
reserved = <x y>]

runs B{y} [active = <0 y>
reserved = <x y>]

[rollover:
active = <0 0>
reserved = <0 y>]

runs C{x} [active = <0 x>]

[resumes]

runs A{x}

At that stage, both A and C have the same ASID, with deadly
consequences.

The fix is to preserve reserved ASIDs across rollovers if
the CPU doesn't have an active ASID when the rollover occurs.

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Carinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm/mm/context.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/arch/arm/mm/context.c
+++ b/arch/arm/mm/context.c
@@ -128,6 +128,15 @@ static void flush_context(unsigned int c
asid = 0;
} else {
asid = atomic64_xchg(&per_cpu(active_asids, i), 0);
+ /*
+ * If this CPU has already been through a
+ * rollover, but hasn't run another task in
+ * the meantime, we must preserve its reserved
+ * ASID, as this is the only trace we have of
+ * the process it is still running.
+ */
+ if (asid == 0)
+ asid = per_cpu(reserved_asids, i);
__set_bit(ASID_TO_IDX(asid), asid_map);
}
per_cpu(reserved_asids, i) = asid;



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