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

[Matt Redfearn <matt.redfearn@mips.com>]

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@mips.com>
Cc: stable <stable@vger.kernel.org> # 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