On 21/12/17 15:30, James Hogan wrote:
On Thu, Dec 21, 2017 at 03:19:35PM +0000, Matt Redfearn wrote:
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 <firstname.lastname@example.org>
Cc: stable <email@example.com> # 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
@@ -760,6 +760,8 @@ static inline void __local_r4k_flush_icache_range(unsigned
+ /* Hazard to force new i-fetch */
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.
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.
Right, but it wouldn't matter since it would still execute valid code?
You'd like to think so, but the Ci40 didn't - that's what led to the
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);
static void r4k_flush_icache_range(unsigned long start, unsigned long end)