• Will Deacon's avatar
    irqchip/gic: Ensure ordering between read of INTACK and shared data · f86c4fbd
    Will Deacon authored
    When an IPI is generated by a CPU, the pattern looks roughly like:
    
      <write shared data>
      smp_wmb();
      <write to GIC to signal SGI>
    
    On the receiving CPU we rely on the fact that, once we've taken the
    interrupt, then the freshly written shared data must be visible to us.
    Put another way, the CPU isn't going to speculate taking an interrupt.
    
    Unfortunately, this assumption turns out to be broken.
    
    Consider that CPUx wants to send an IPI to CPUy, which will cause CPUy
    to read some shared_data. Before CPUx has done anything, a random
    peripheral raises an IRQ to the GIC and the IRQ line on CPUy is raised.
    CPUy then takes the IRQ and starts executing the entry code, heading
    towards gic_handle_irq. Furthermore, let's assume that a bunch of the
    previous interrupts handled by CPUy were SGIs, so the branch predictor
    kicks in and speculates that irqnr will be <16 and we're likely to
    head into handle_IPI. The prefetcher then grabs a speculative copy of
    shared_data which contains a stale value.
    
    Meanwhile, CPUx gets round to updating shared_data and asking the GIC
    to send an SGI to CPUy. Internally, the GIC decides that the SGI is
    more important than the peripheral interrupt (which hasn't yet been
    ACKed) but doesn't need to do anything to CPUy, because the IRQ line
    is already raised.
    
    CPUy then reads the ACK register on the GIC, sees the SGI value which
    confirms the branch prediction and we end up with a stale shared_data
    value.
    
    This patch fixes the problem by adding an smp_rmb() to the IPI entry
    code in gic_handle_irq. As it turns out, the combination of a control
    dependency and an ISB instruction from the EOI in the GICv3 driver is
    enough to provide the ordering we need, so we add a comment there
    justifying the absence of an explicit smp_rmb().
    
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    f86c4fbd
irq-gic.c 35.7 KB