Re: [PATCH V2 2/2] MIPS: irqflags.h: make funcs preempt-safe for non-mipsr2

[David Daney <ddaney.cavm@gmail.com>]

On 08/29/2012 03:34 PM, Jim Quinlan wrote:
> For non MIPSr2 processors, such as the BMIPS 5000, calls to
> arch_local_irq_disable() and others may be preempted, and in doing
> so a stale value may be restored to c0_status.  This fix disables
> preemption for such processors prior to the call and enables it
> after the call.
>
> This bug was observed in a BMIPS 5000, occuring once every few hours
> in a continuous reboot test.  It was traced to the write_lock_irq()
> function which was being invoked in release_task() in exit.c.
> By placing a number of "nops" inbetween the mfc0/mtc0 pair in
> arch_local_irq_disable(), which is called by write_lock_irq(), we
> were able to greatly increase the occurance of this bug.  Similarly,
> the application of this commit silenced the bug.
>
> It is better to use the preemption functions declared in <linux/preempt.h>
> rather than defining new ones as is done in this commit.  However,
> including that file from irqflags effected many compiler errors.
>
> Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
> ---
>   arch/mips/include/asm/irqflags.h |   81 ++++++++++++++++++++++++++++++++++++++
>   1 files changed, 81 insertions(+), 0 deletions(-)
>
> diff --git a/arch/mips/include/asm/irqflags.h b/arch/mips/include/asm/irqflags.h
> index 309cbcd..d6e71ed 100644
> --- a/arch/mips/include/asm/irqflags.h
> +++ b/arch/mips/include/asm/irqflags.h
> @@ -16,6 +16,71 @@
>   #include <linux/compiler.h>
>   #include <asm/hazards.h>
>
> +#if defined(__GENERATING_BOUNDS_H) || defined(__GENERATING_OFFSETS_S)
> +#define __TI_PRE_COUNT (-1)
> +#else
> +#include <asm/asm-offsets.h>
> +#define __TI_PRE_COUNT TI_PRE_COUNT
> +#endif
> +
> +
> +/*
> + * Non-mipsr2 processors executing functions such as arch_local_irq_disable()
> + * are not preempt-safe: if preemption occurs between the mfc0 and the mtc0,
> + * a stale status value may be stored.  To prevent this, we define
> + * here arch_local_preempt_disable() and arch_local_preempt_enable(), which
> + * are called before the mfc0 and after the mtc0, respectively.  A better
> + * solution would "#include <linux/preempt.h> and use its declared routines,
> + * but that is not viable due to numerous compile errors.
> + *

I'm with Ralf's idea from the other branch of the thread.  Put all this 
non-mipsr2 stuff out of line (perhaps creating lib/mips-atomic.c).


> + * MipsR2 processors with atomic interrupt enable/disable instructions
> + * (ei/di) do not have this issue.
> + */

For mipsr2, we leave them alone so they can be inlined.

This way you shouldn't need the ugly #include hackery.

David Daney