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

[Mark Rutland <mark.rutland@arm.com>]

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**********************************/