Re: [PATCH 1/3] MIPS: c-r4k: instruction_hazard should immediately follow cache op

[Matt Redfearn]

Hi James,

On 21/12/17 15:14, James Hogan wrote:
> On Thu, Dec 21, 2017 at 11:16:02AM +0000, Matt Redfearn wrote:
> > During ftrace initialisation, placeholder instructions in the prologue
> > of every kernel function not marked "notrace" are replaced with nops.
> > After the instructions are written (to the dcache), flush_icache_range()
> > is used to ensure that the icache will be updated with these replaced
> > instructions. Currently there is an instruction_hazard guard at the end
> > of __r4k_flush_icache_range, since a hazard can be created if the CPU
> > has already begun fetching the instructions that have have been
> > replaced. The placement, however, ignores the calls to preempt_enable(),
> > both in __r4k_flush_icache_range and r4k_on_each_cpu. When
> > CONFIG_PREEMPT is enabled, these expand out to at least calls to
> > preempt_count_sub(). The lack of an instruction hazard between icache
> > invalidate and the execution of preempt_count_sub, in rare
> > circumstances, was observed to cause weird crashes on Ci40, where the
> > CPU would end up taking a kernel unaligned access exception from the
> > middle of do_ade(), which it somehow reached from preempt_count_sub
> > without executing the start of do_ade.
> >
> > Since the instruction hazard exists immediately after the dcache is
> > written back and icache invalidated, place the instruction_hazard()
> > within __local_r4k_flush_icache_range. The one at the end of
> > __r4k_flush_icache_range is too late, since all of the functions in the
> > call path of preempt_enable have already been executed, so remove it.
> >
> > This fixes the crashes during ftrace initialisation on Ci40.
> >
> > Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxx>
> > Cc: stable <stable@xxxxxxxxxxxxxxx> # v4.9+
> >
> > ---
> >
> >   arch/mips/mm/c-r4k.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/mm/c-r4k.c b/arch/mips/mm/c-r4k.c
> > index 6f534b209971..ce7a54223504 100644
> > --- a/arch/mips/mm/c-r4k.c
> > +++ b/arch/mips/mm/c-r4k.c
> > @@ -760,6 +760,8 @@ static inline void __local_r4k_flush_icache_range(unsigned long start,
> >   			break;
> >   		}
> >   	}
> > +	/* Hazard to force new i-fetch */
> > +	instruction_hazard();
>
> By the sounds of it that is a hardware bug, that it didn't try and
> execute either the old instruction or the new instruction.

Yeah, possibly.

 Maybe an
> expanded comment would be worthwhile here. If it wasn't for that issue
> it would I suppose be safe for it to be directly before the
> preempt_enable() in __r4k_flush_icache_range().

No - there's another preempt_enable() in r4k_on_each_cpu (noted in the 
commit message) so by the time the local CPU gets to the 
preempt_enable() in __r4k_flush_icache_range, it has potentially already 
executed the preempt_enable path and died. That's why I put it here.

Thanks,
Matt

> Cheers
> James
>
> >   }
> >   
> >   static inline void local_r4k_flush_icache_range(unsigned long start,
> > @@ -817,7 +819,6 @@ static void __r4k_flush_icache_range(unsigned long start, unsigned long end,
> >   	}
> >   	r4k_on_each_cpu(args.type, local_r4k_flush_icache_range_ipi, &args);
> >   	preempt_enable();
> > -	instruction_hazard();
> >   }
> >   
> >   static void r4k_flush_icache_range(unsigned long start, unsigned long end)
> > --
> > 2.7.4