Messages in this thread Patch in this message | | | Date | Thu, 30 May 2019 09:38:47 +0800 | From | tengfeif@codeauro ... | Subject | Re: [PATCH] arm64: break while loop if task had been rescheduled |
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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: …… { 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().
> > 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@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; > > 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).
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