Messages in this thread Patch in this message | | | Subject | Re: [PATCH] arm64: break while loop if task had been rescheduled | From | Anshuman Khandual <> | Date | Wed, 22 May 2019 14:34:46 +0530 |
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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.
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.
> > 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.
> } > #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 uninterruptible to > + * running before hitting the do-while loop but after > + * saving the current state. If task is in running
This does not check any explicit task states like 'un-interruptible' or 'running' but instead tracks change from any previously 'saved' state.
> + * state before saving the 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.
> + * will stop print when that task schedule out. 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.
> + */ > + || 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 ?
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