[PATCH] Kprobes: preempt_disable/enable() simplification

Reorganize the preempt_disable/enable calls to eliminate the extra preempt
depth.  Changes based on Paul McKenney's review suggestions for the kprobes
RCU changeset.
Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/kprobes.c

@@ -153,7 +153,14 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 	int ret = 0;
 	kprobe_opcode_t *addr = NULL;
 	unsigned long *lp;
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	struct kprobe_ctlblk *kcb;
+
+	/*
+	 * We don't want to be preempted for the entire
+	 * duration of kprobe processing
+	 */
+	preempt_disable();
+	kcb = get_kprobe_ctlblk();
 
 	/* Check if the application is using LDT entry for its code segment and
 	 * calculate the address by reading the base address from the LDT entry.
@@ -221,11 +228,6 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 		goto no_kprobe;
 	}
 
-	/*
-	 * This preempt_disable() matches the preempt_enable_no_resched()
-	 * in post_kprobe_handler()
-	 */
-	preempt_disable();
 	set_current_kprobe(p, regs, kcb);
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 
@@ -239,6 +241,7 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 	return 1;
 
 no_kprobe:
+	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -310,8 +313,8 @@ int __kprobes trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
 
 	/*
 	 * By returning a non-zero value, we are telling
-	 * kprobe_handler() that we have handled unlocking
-	 * and re-enabling preemption
+	 * kprobe_handler() that we don't want the post_handler
+	 * to run (and have re-enabled preemption)
 	 */
         return 1;
 }
@@ -455,7 +458,6 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	struct die_args *args = (struct die_args *)data;
 	int ret = NOTIFY_DONE;
 
-	rcu_read_lock();
 	switch (val) {
 	case DIE_INT3:
 		if (kprobe_handler(args->regs))
@@ -467,14 +469,16 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 		break;
 	case DIE_GPF:
 	case DIE_PAGE_FAULT:
+		/* kprobe_running() needs smp_processor_id() */
+		preempt_disable();
 		if (kprobe_running() &&
 		    kprobe_fault_handler(args->regs, args->trapnr))
 			ret = NOTIFY_STOP;
+		preempt_enable();
 		break;
 	default:
 		break;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -537,6 +541,7 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 		*regs = kcb->jprobe_saved_regs;
 		memcpy((kprobe_opcode_t *) stack_addr, kcb->jprobes_stack,
 		       MIN_STACK_SIZE(stack_addr));
+		preempt_enable_no_resched();
 		return 1;
 	}
 	return 0;

arch/ia64/kernel/kprobes.c

@@ -391,8 +391,8 @@ int __kprobes trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
 
 	/*
 	 * By returning a non-zero value, we are telling
-         * kprobe_handler() that we have handled unlocking
-	 * and re-enabling preemption
+	 * kprobe_handler() that we don't want the post_handler
+	 * to run (and have re-enabled preemption)
 	 */
         return 1;
 }
@@ -604,7 +604,14 @@ static int __kprobes pre_kprobes_handler(struct die_args *args)
 	int ret = 0;
 	struct pt_regs *regs = args->regs;
 	kprobe_opcode_t *addr = (kprobe_opcode_t *)instruction_pointer(regs);
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	struct kprobe_ctlblk *kcb;
+
+	/*
+	 * We don't want to be preempted for the entire
+	 * duration of kprobe processing
+	 */
+	preempt_disable();
+	kcb = get_kprobe_ctlblk();
 
 	/* Handle recursion cases */
 	if (kprobe_running()) {
@@ -659,11 +666,6 @@ static int __kprobes pre_kprobes_handler(struct die_args *args)
 		goto no_kprobe;
 	}
 
-	/*
-	 * This preempt_disable() matches the preempt_enable_no_resched()
-	 * in post_kprobes_handler()
-	 */
-	preempt_disable();
 	set_current_kprobe(p, kcb);
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 
@@ -681,6 +683,7 @@ static int __kprobes pre_kprobes_handler(struct die_args *args)
 	return 1;
 
 no_kprobe:
+	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -716,9 +719,6 @@ static int __kprobes kprobes_fault_handler(struct pt_regs *regs, int trapnr)
 	struct kprobe *cur = kprobe_running();
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 
-	if (!cur)
-		return 0;
-
 	if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
 		return 1;
 
@@ -737,7 +737,6 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	struct die_args *args = (struct die_args *)data;
 	int ret = NOTIFY_DONE;
 
-	rcu_read_lock();
 	switch(val) {
 	case DIE_BREAK:
 		if (pre_kprobes_handler(args))
@@ -748,12 +747,15 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 			ret = NOTIFY_STOP;
 		break;
 	case DIE_PAGE_FAULT:
-		if (kprobes_fault_handler(args->regs, args->trapnr))
+		/* kprobe_running() needs smp_processor_id() */
+		preempt_disable();
+		if (kprobe_running() &&
+			kprobes_fault_handler(args->regs, args->trapnr))
 			ret = NOTIFY_STOP;
+		preempt_enable();
 	default:
 		break;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -785,6 +787,7 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 
 	*regs = kcb->jprobe_saved_regs;
+	preempt_enable_no_resched();
 	return 1;
 }
 

arch/ppc64/kernel/kprobes.c

@@ -148,7 +148,14 @@ static inline int kprobe_handler(struct pt_regs *regs)
 	struct kprobe *p;
 	int ret = 0;
 	unsigned int *addr = (unsigned int *)regs->nip;
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	struct kprobe_ctlblk *kcb;
+
+	/*
+	 * We don't want to be preempted for the entire
+	 * duration of kprobe processing
+	 */
+	preempt_disable();
+	kcb = get_kprobe_ctlblk();
 
 	/* Check we're not actually recursing */
 	if (kprobe_running()) {
@@ -207,11 +214,6 @@ static inline int kprobe_handler(struct pt_regs *regs)
 		goto no_kprobe;
 	}
 
-	/*
-	 * This preempt_disable() matches the preempt_enable_no_resched()
-	 * in post_kprobe_handler().
-	 */
-	preempt_disable();
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 	set_current_kprobe(p, regs, kcb);
 	if (p->pre_handler && p->pre_handler(p, regs))
@@ -224,6 +226,7 @@ static inline int kprobe_handler(struct pt_regs *regs)
 	return 1;
 
 no_kprobe:
+	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -296,8 +299,8 @@ int __kprobes trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
 
         /*
          * By returning a non-zero value, we are telling
-         * kprobe_handler() that we have handled unlocking
-         * and re-enabling preemption.
+         * kprobe_handler() that we don't want the post_handler
+         * to run (and have re-enabled preemption)
          */
         return 1;
 }
@@ -385,7 +388,6 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	struct die_args *args = (struct die_args *)data;
 	int ret = NOTIFY_DONE;
 
-	rcu_read_lock();
 	switch (val) {
 	case DIE_BPT:
 		if (kprobe_handler(args->regs))
@@ -396,14 +398,16 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 			ret = NOTIFY_STOP;
 		break;
 	case DIE_PAGE_FAULT:
+		/* kprobe_running() needs smp_processor_id() */
+		preempt_disable();
 		if (kprobe_running() &&
 		    kprobe_fault_handler(args->regs, args->trapnr))
 			ret = NOTIFY_STOP;
+		preempt_enable();
 		break;
 	default:
 		break;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -440,6 +444,7 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 	 * saved regs...
 	 */
 	memcpy(regs, &kcb->jprobe_saved_regs, sizeof(struct pt_regs));
+	preempt_enable_no_resched();
 	return 1;
 }
 

arch/sparc64/kernel/kprobes.c

@@ -113,7 +113,14 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 	struct kprobe *p;
 	void *addr = (void *) regs->tpc;
 	int ret = 0;
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	struct kprobe_ctlblk *kcb;
+
+	/*
+	 * We don't want to be preempted for the entire
+	 * duration of kprobe processing
+	 */
+	preempt_disable();
+	kcb = get_kprobe_ctlblk();
 
 	if (kprobe_running()) {
 		p = get_kprobe(addr);
@@ -159,11 +166,6 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 		goto no_kprobe;
 	}
 
-	/*
-	 * This preempt_disable() matches the preempt_enable_no_resched()
-	 * in post_kprobes_handler()
-	 */
-	preempt_disable();
 	set_current_kprobe(p, regs, kcb);
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 	if (p->pre_handler && p->pre_handler(p, regs))
@@ -175,6 +177,7 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
 	return 1;
 
 no_kprobe:
+	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -321,7 +324,6 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	struct die_args *args = (struct die_args *)data;
 	int ret = NOTIFY_DONE;
 
-	rcu_read_lock();
 	switch (val) {
 	case DIE_DEBUG:
 		if (kprobe_handler(args->regs))
@@ -333,14 +335,16 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 		break;
 	case DIE_GPF:
 	case DIE_PAGE_FAULT:
+		/* kprobe_running() needs smp_processor_id() */
+		preempt_disable();
 		if (kprobe_running() &&
 		    kprobe_fault_handler(args->regs, args->trapnr))
 			ret = NOTIFY_STOP;
+		preempt_enable();
 		break;
 	default:
 		break;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -426,6 +430,7 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 		       &(kcb->jprobe_saved_stack),
 		       sizeof(kcb->jprobe_saved_stack));
 
+		preempt_enable_no_resched();
 		return 1;
 	}
 	return 0;

arch/x86_64/kernel/kprobes.c

@@ -286,16 +286,19 @@ void __kprobes arch_prepare_kretprobe(struct kretprobe *rp,
         }
 }
 
-/*
- * Interrupts are disabled on entry as trap3 is an interrupt gate and they
- * remain disabled thorough out this function.
- */
 int __kprobes kprobe_handler(struct pt_regs *regs)
 {
 	struct kprobe *p;
 	int ret = 0;
 	kprobe_opcode_t *addr = (kprobe_opcode_t *)(regs->rip - sizeof(kprobe_opcode_t));
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
+	struct kprobe_ctlblk *kcb;
+
+	/*
+	 * We don't want to be preempted for the entire
+	 * duration of kprobe processing
+	 */
+	preempt_disable();
+	kcb = get_kprobe_ctlblk();
 
 	/* Check we're not actually recursing */
 	if (kprobe_running()) {
@@ -359,11 +362,6 @@ int __kprobes kprobe_handler(struct pt_regs *regs)
 		goto no_kprobe;
 	}
 
-	/*
-	 * This preempt_disable() matches the preempt_enable_no_resched()
-	 * in post_kprobe_handler()
-	 */
-	preempt_disable();
 	set_current_kprobe(p, regs, kcb);
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 
@@ -377,6 +375,7 @@ int __kprobes kprobe_handler(struct pt_regs *regs)
 	return 1;
 
 no_kprobe:
+	preempt_enable_no_resched();
 	return ret;
 }
 
@@ -448,8 +447,8 @@ int __kprobes trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
 
         /*
          * By returning a non-zero value, we are telling
-         * kprobe_handler() that we have handled unlocking
-	 * and re-enabling preemption
+         * kprobe_handler() that we don't want the post_handler
+	 * to run (and have re-enabled preemption)
          */
         return 1;
 }
@@ -594,7 +593,6 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 	struct die_args *args = (struct die_args *)data;
 	int ret = NOTIFY_DONE;
 
-	rcu_read_lock();
 	switch (val) {
 	case DIE_INT3:
 		if (kprobe_handler(args->regs))
@@ -606,14 +604,16 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 		break;
 	case DIE_GPF:
 	case DIE_PAGE_FAULT:
+		/* kprobe_running() needs smp_processor_id() */
+		preempt_disable();
 		if (kprobe_running() &&
 		    kprobe_fault_handler(args->regs, args->trapnr))
 			ret = NOTIFY_STOP;
+		preempt_enable();
 		break;
 	default:
 		break;
 	}
-	rcu_read_unlock();
 	return ret;
 }
 
@@ -675,6 +675,7 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 		*regs = kcb->jprobe_saved_regs;
 		memcpy((kprobe_opcode_t *) stack_addr, kcb->jprobes_stack,
 		       MIN_STACK_SIZE(stack_addr));
+		preempt_enable_no_resched();
 		return 1;
 	}
 	return 0;

include/linux/kprobes.h

@@ -159,7 +159,7 @@ extern void show_registers(struct pt_regs *regs);
 extern kprobe_opcode_t *get_insn_slot(void);
 extern void free_insn_slot(kprobe_opcode_t *slot);
 
-/* Get the kprobe at this addr (if any) - called under a rcu_read_lock() */
+/* Get the kprobe at this addr (if any) - called with preemption disabled */
 struct kprobe *get_kprobe(void *addr);
 struct hlist_head * kretprobe_inst_table_head(struct task_struct *tsk);
 

kernel/kprobes.c

@@ -167,7 +167,7 @@ static inline void reset_kprobe_instance(void)
  * This routine is called either:
  * 	- under the kprobe_lock spinlock - during kprobe_[un]register()
  * 				OR
- * 	- under an rcu_read_lock() - from arch/xxx/kernel/kprobes.c
+ * 	- with preemption disabled - from arch/xxx/kernel/kprobes.c
  */
 struct kprobe __kprobes *get_kprobe(void *addr)
 {