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(); } 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