| From | Greg Kroah-Hartman <> | Subject | [ 60/72] ARM: 7767/1: let the ASID allocator handle suspended animation | Date | Thu, 18 Jul 2013 22:26:17 -0700 |
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3.10-stable review patch. If anyone has any objections, please let me know.
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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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