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SubjectRe: [PATCH] arm64: break while loop if task had been rescheduled
Hi,

This appears to be a bizarrely formatted reply to Anshuman's questions
[1] on the first posting [2] of this patch, and as it stands, it isn't
possible to follow.

Please follow the usual mailing list ettiquette, and reply inline to
questions.

I am not going to reply further to this post, but I'll comment on the
first post.

Thanks,
Mark.

[1] https://lore.kernel.org/lkml/1558430404-4840-1-git-send-email-tengfeif@codeaurora.org/T/#m415174aacdd100f9386113ed3ae9f427a2255f8a
[2] https://lore.kernel.org/lkml/1558430404-4840-1-git-send-email-tengfeif@codeaurora.org/T/#u

On Fri, May 24, 2019 at 11:16:16AM +0800, tengfeif@codeaurora.org wrote:
> When task isn't current task, this task's state have
> chance to be changed during printing this task's
> backtrace, so it is possible that task's fp and fp+8
> have the same vaule, so cannot break the while loop.
> To fix this issue, we first save the task's state, sp
> and fp, then we will get the task's current state, sp
> and fp in each while again. we will stop to print
> backtrace if we found any of the values are different
> than what we saved.
>
> /********************************answer
> question**********************************/
> 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.

So

>
> 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:
> ……
> {
> 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]
> ……
> 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().
>
> /********************************answer
> question**********************************/
>
> /********************************answer
> question**********************************/
> 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.
> /********************************answer
> question**********************************/
>
> Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>
> ---
> 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;
>
> /********************************answer
> question**********************************/
> Should 'saved_state|sp|fp' instead as its applicable to non-current
> tasks only.
> 'saved_state|sp|fp' only applies to non-current tasks.
>
> /********************************answer
> question**********************************/
>
> }
> #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 ohter
> + * state before hitting the do-while loop but after
> + * saving the current state. If task's current state
> + * 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
> + * 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.
> + */
> + || cur_sp != thread_saved_sp(tsk)
> + || cur_fp != thread_saved_fp(tsk))) {
>
> /********************************answer
> question**********************************/
> 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).
> /********************************answer
> question**********************************/

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