Re: [PATCH] arm64: break while loop if task had been rescheduled

From: tengfeif
Date: Wed May 29 2019 - 21:42:20 EST


On 2019-05-22 17:04, Anshuman Khandual wrote:
On 05/21/2019 02:50 PM, Tengfei Fan wrote:
While printing a task's backtrace and this task isn't
current task, it is possible that task's fp and fp+8
have the same value, so cannot break the while loop.
This can break while loop if this task had been
rescheduled during print this task's backtrace.

This is very confusing. IIUC it suggests that while printing
the backtrace for non-current tasks the do/while loop does not
exit because fp and fp+8 might have the same value ? When would
this happen ? Even in that case the commit message here does not
properly match the change in this patch.

In our issue, we got fp=pc=0xFFFFFF8025A13BA0, so cannot exit while
loop in dump_basktrace().
After analyze our issue's dump, we found one task(such as: task A)
is exiting via invoke do_exit() during another task is showing task
A's dumptask. In kernel code, do_exit() and exit_notify are defined
as follows:
void noreturn do_exit(long code)
{
......
exit_notify(tsk, group_dead);
......
}
static void exit_notify(struct task_struct *tsk, int group_dead)
{
......
}
Because of exit_notify() is a static function, so it is inlined to
do_exit() when compile kernel, so we can get partial assembly code
of do_exit() as follows:
ââ
{
bool autoreap;
struct task_struct *p, *n;
LIST_HEAD(dead);

write_lock_irq(&tasklist_lock);
c10: 90000000 adrp x0, 0 <tasklist_lock>
c14: 910003e8 mov x8, sp
c18: 91000000 add x0, x0, #0x0
*/
static void exit_notify(struct task_struct *tsk, int group_dead)
{
bool autoreap;
struct task_struct *p, *n;
LIST_HEAD(dead);
c1c: a90023e8 stp x8, x8, [sp]

write_lock_irq(&tasklist_lock);
c20: 94000000 bl 0 <_raw_write_lock_irq>
c24: f9435268 ldr x8, [x19,#1696]
ââ
From the code "c14:" and "c1c:", we will find sp's addr value is stored
in sp and sp+8, so sp's vaule equal (sp+8)'s value.
In our issue, there is a chance of fp point sp, so there will be fp=pc=fp's
addr value,so code cannot break from while loop in dump_backtrace().


This patch tries to stop printing the stack for non-current tasks
if their state change while there is one dump_backtrace() trying
to print back trace. Dont we have any lock preventing a task in
this situation (while dumping it's backtrace) from running again
or changing state.

I haven't found any lock preventing a task in this situation, and I think we shouldn't
prevent task running if this task is scheduled.


Signed-off-by: Tengfei Fan <tengfeif@xxxxxxxxxxxxxx>
---
arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 2975598..9df6e02 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
{
struct stackframe frame;
int skip = 0;
+ long cur_state = 0;
+ unsigned long cur_sp = 0;
+ unsigned long cur_fp = 0;

pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);

@@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
*/
frame.fp = thread_saved_fp(tsk);
frame.pc = thread_saved_pc(tsk);
+ cur_state = tsk->state;
+ cur_sp = thread_saved_sp(tsk);
+ cur_fp = frame.fp;

Should 'saved_state|sp|fp' instead as its applicable to non-current
tasks only.

'saved_state|sp|fp' only applies to non-current tasks.


}
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
frame.graph = 0;
@@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)

printk("Call trace:\n");
do {
+ if (tsk != current && (cur_state != tsk->state
+ /*
+ * We would not be printing backtrace for the task
+ * that has changed state from "saved" state to other
+ * state before hitting the do-while loop but after
+ * saving the current state. If task's current state

This does not check any explicit task states like 'un-interruptible' or
'running' but instead tracks change from any previously 'saved' state.

have updated comments.


+ * not equal the "saved" state, then we may print
+ * wrong call trace or end up in infinite while loop
+ * if *(fp) and *(fp+8) are same. While the situation

Then dump_backtrace() must detect it, should not save it and just abort.

have updatd commentes.


+ * should be stoped once we found the task's state
+ * is changed, so we detect the task's current state,
+ * sp and fp in each while.

Thats not a reliable solution. AFICS we should not proceed further if
there is a chance of an wrong trace or an infinite loop. Hoping that
the printing will stop when task gets scheduled out does not seem right.

In this patch, it will break while loop and stop to print backtrace if we
find the task's state change or there is a chance of an infinite loop.

+ */
+ || cur_sp != thread_saved_sp(tsk)
+ || cur_fp != thread_saved_fp(tsk))) {

Why does any of these three mismatches detect the problematic transition
not just the state ?

1. we can use "cur_state != tsk->state" prevent printing backtrace if the task's
state is changed after "saved" task's state.
2. we can use "cur_sp != thread_saved_sp(tsk)" and "cur_fp != thread_saved_fp(tsk)"
prevent printing backtrace if the task's state is changed before "saved" task's
state. Because the value of "thread_saved_sp(tsk)" and "thread_saved_fp(tsk)"
will not equal "saved" sp(cur_sp) and fp(cur_fp).