ARM: 7767/1: let the ASID allocator handle suspended animation
authorMarc Zyngier <Marc.Zyngier@arm.com>
Fri, 21 Jun 2013 11:06:19 +0000 (12:06 +0100)
committerRussell King <rmk+kernel@arm.linux.org.uk>
Mon, 24 Jun 2013 14:24:54 +0000 (15:24 +0100)
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.

Cc: <stable@vger.kernel.org> # 3.9
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>
arch/arm/mm/context.c

index 2ac3737..8e12fcb 100644 (file)
@@ -128,6 +128,15 @@ static void flush_context(unsigned int cpu)
                        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;