[MIPS] Fix potential latency problem due to non-atomic cpu_wait.

If an interrupt happened between checking of NEED_RESCHED and WAIT
instruction, adjust EPC to restart from checking of NEED_RESCHED.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

3 files changed, 55 insertions, 20 deletions

diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index 335a6ae3d594..11c92dc53791 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -45,18 +45,7 @@ static void r39xx_wait(void)
         local_irq_enable();
 }
 
-/*
- * There is a race when WAIT instruction executed with interrupt
- * enabled.
- * But it is implementation-dependent wheter the pipelie restarts when
- * a non-enabled interrupt is requested.
- */
-static void r4k_wait(void)
-{
-        __asm__("       .set    mips3                   \n"
-                "       wait                            \n"
-                "       .set    mips0                   \n");
-}
+extern void r4k_wait(void);
 
 /*
  * This variant is preferable as it allows testing need_resched and going to
@@ -128,7 +117,7 @@ static int __init wait_disable(char *s)
 
 __setup("nowait", wait_disable);
 
-static inline void check_wait(void)
+void __init check_wait(void)
 {
         struct cpuinfo_mips *c = &current_cpu_data;
 
@@ -242,7 +231,6 @@ static inline void check_errata(void)
 
 void __init check_bugs32(void)
 {
-        check_wait();
         check_errata();
 }
 
diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index c6ada98ee042..f886dd7f708e 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -20,6 +20,7 @@
 #include <asm/stackframe.h>
 #include <asm/war.h>
 #include <asm/page.h>
+#include <asm/thread_info.h>
 
 #define PANIC_PIC(msg)                                  \
                 .set push;                              \
@@ -126,7 +127,42 @@ handle_vcei:
 
         __FINIT
 
+        .align  5       /* 32 byte rollback region */
+LEAF(r4k_wait)
+        .set    push
+        .set    noreorder
+        /* start of rollback region */
+        LONG_L  t0, TI_FLAGS($28)
+        nop
+        andi    t0, _TIF_NEED_RESCHED
+        bnez    t0, 1f
+         nop
+        nop
+        nop
+        .set    mips3
+        wait
+        /* end of rollback region (the region size must be power of two) */
+        .set    pop
+1:
+        jr      ra
+        END(r4k_wait)
+
+        .macro  BUILD_ROLLBACK_PROLOGUE handler
+        FEXPORT(rollback_\handler)
+        .set    push
+        .set    noat
+        MFC0    k0, CP0_EPC
+        PTR_LA  k1, r4k_wait
+        ori     k0, 0x1f        /* 32 byte rollback region */
+        xori    k0, 0x1f
+        bne     k0, k1, 9f
+        MTC0    k0, CP0_EPC
+9:
+        .set pop
+        .endm
+
         .align  5
+BUILD_ROLLBACK_PROLOGUE handle_int
 NESTED(handle_int, PT_SIZE, sp)
 #ifdef CONFIG_TRACE_IRQFLAGS
         /*
@@ -201,6 +237,7 @@ NESTED(except_vec_ejtag_debug, 0, sp)
  * This prototype is copied to ebase + n*IntCtl.VS and patched
  * to invoke the handler
  */
+BUILD_ROLLBACK_PROLOGUE except_vec_vi
 NESTED(except_vec_vi, 0, sp)
         SAVE_SOME
         SAVE_AT
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 6bee29097a56..5fd0cd020af5 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -46,6 +46,9 @@
 #include <asm/types.h>
 #include <asm/stacktrace.h>
 
+extern void check_wait(void);
+extern asmlinkage void r4k_wait(void);
+extern asmlinkage void rollback_handle_int(void);
 extern asmlinkage void handle_int(void);
 extern asmlinkage void handle_tlbm(void);
 extern asmlinkage void handle_tlbl(void);
@@ -1251,6 +1254,9 @@ static void *set_vi_srs_handler(int n, vi_handler_t addr, int srs)
 
                 extern char except_vec_vi, except_vec_vi_lui;
                 extern char except_vec_vi_ori, except_vec_vi_end;
+                extern char rollback_except_vec_vi;
+                char *vec_start = (cpu_wait == r4k_wait) ?
+                        &rollback_except_vec_vi : &except_vec_vi;
 #ifdef CONFIG_MIPS_MT_SMTC
                 /*
                  * We need to provide the SMTC vectored interrupt handler
@@ -1258,11 +1264,11 @@ static void *set_vi_srs_handler(int n, vi_handler_t addr, int srs)
                  * Status.IM bit to be masked before going there.
                  */
                 extern char except_vec_vi_mori;
-                const int mori_offset = &except_vec_vi_mori - &except_vec_vi;
+                const int mori_offset = &except_vec_vi_mori - vec_start;
 #endif /* CONFIG_MIPS_MT_SMTC */
-                const int handler_len = &except_vec_vi_end - &except_vec_vi;
-                const int lui_offset = &except_vec_vi_lui - &except_vec_vi;
-                const int ori_offset = &except_vec_vi_ori - &except_vec_vi;
+                const int handler_len = &except_vec_vi_end - vec_start;
+                const int lui_offset = &except_vec_vi_lui - vec_start;
+                const int ori_offset = &except_vec_vi_ori - vec_start;
 
                 if (handler_len > VECTORSPACING) {
                         /*
@@ -1272,7 +1278,7 @@ static void *set_vi_srs_handler(int n, vi_handler_t addr, int srs)
                         panic("VECTORSPACING too small");
                 }
 
-                memcpy(b, &except_vec_vi, handler_len);
+                memcpy(b, vec_start, handler_len);
 #ifdef CONFIG_MIPS_MT_SMTC
                 BUG_ON(n > 7);  /* Vector index %d exceeds SMTC maximum. */
 
@@ -1554,6 +1560,10 @@ void __init trap_init(void)
         extern char except_vec3_generic, except_vec3_r4000;
         extern char except_vec4;
         unsigned long i;
+        int rollback;
+
+        check_wait();
+        rollback = (cpu_wait == r4k_wait);
 
 #if defined(CONFIG_KGDB)
         if (kgdb_early_setup)
@@ -1618,7 +1628,7 @@ void __init trap_init(void)
         if (board_be_init)
                 board_be_init();
 
-        set_except_vector(0, handle_int);
+        set_except_vector(0, rollback ? rollback_handle_int : handle_int);
         set_except_vector(1, handle_tlbm);
         set_except_vector(2, handle_tlbl);
         set_except_vector(3, handle_tlbs);