On Fri, Oct 22, 2021 at 07:01:35PM +0200, Peter Zijlstra wrote: > On Fri, Oct 22, 2021 at 05:54:31PM +0100, Mark Rutland wrote: > > > > Pardon my thin understanding of the scheduler, but I assume this change > > > doesn't mean stack_trace_save_tsk() stops working for "current", right? > > > In trying to answer this for myself, I couldn't convince myself what value > > > current->__state have here. Is it one of TASK_(UN)INTERRUPTIBLE ? > > > > Regardless of that, current->on_rq will be non-zero, so you're right that this > > causes stack_trace_save_tsk() to not work for current, e.g. > > > > | # cat /proc/self/stack > > | # wc /proc/self/stack > > | 0 0 0 /proc/self/stack > > > > TBH, I think that (taking a step back from this issue in particular) > > stack_trace_save_tsk() *shouldn't* work for current, and callers *should* be > > forced to explicitly handle current separately from blocked tasks. > > That.. So I think I'd prefer the following approach to that (and i'm not currently volunteering for it): - convert all archs to ARCH_STACKWALK; this gets the semantics out of arch code and into the single kernel/stacktrace.c file. - bike-shed a new/improved stack_trace_save*() API and implement it *once* in generic code based on arch_stack_walk(). - convert users; delete old etc.. For now, current users of stack_trace_save_tsk() very much expect tsk==current to work. > > So we could fix this in the stacktrace code with: > > > > | diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c > > | index a1cdbf8c3ef8..327af9ff2c55 100644 > > | --- a/kernel/stacktrace.c > > | +++ b/kernel/stacktrace.c > > | @@ -149,7 +149,10 @@ unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store, > > | .skip = skipnr + (current == tsk), > > | }; > > | > > | - task_try_func(tsk, try_arch_stack_walk_tsk, &c); > > | + if (tsk == current) > > | + try_arch_stack_walk_tsk(tsk, &c); > > | + else > > | + task_try_func(tsk, try_arch_stack_walk_tsk, &c); > > | > > | return c.len; > > | } > > > > ... and we could rename task_try_func() to blocked_task_try_func(), and > > later push the distinction into higher-level callers. > > I think I favour this fix if we have to. But that's for next week :-) I ended up with the below delta to this patch. --- a/kernel/stacktrace.c +++ b/kernel/stacktrace.c @@ -101,7 +101,7 @@ static bool stack_trace_consume_entry_no } /** - * stack_trace_save - Save a stack trace into a storage array + * stack_trace_save - Save a stack trace (of current) into a storage array * @store: Pointer to storage array * @size: Size of the storage array * @skipnr: Number of entries to skip at the start of the stack trace @@ -132,7 +132,7 @@ static int try_arch_stack_walk_tsk(struc /** * stack_trace_save_tsk - Save a task stack trace into a storage array - * @task: The task to examine + * @task: The task to examine (current allowed) * @store: Pointer to storage array * @size: Size of the storage array * @skipnr: Number of entries to skip at the start of the stack trace @@ -149,13 +149,25 @@ unsigned int stack_trace_save_tsk(struct .skip = skipnr + (current == tsk), }; - task_try_func(tsk, try_arch_stack_walk_tsk, &c); + /* + * If the task doesn't have a stack (e.g., a zombie), the stack is + * empty. + */ + if (!try_get_task_stack(tsk)) + return 0; + + if (tsk == current) + try_arch_stack_walk_tsk(tsk, &c); + else + task_try_func(tsk, try_arch_stack_walk_tsk, &c); + + put_task_stack(tsk); return c.len; } /** - * stack_trace_save_regs - Save a stack trace based on pt_regs into a storage array + * stack_trace_save_regs - Save a stack trace (of current) based on pt_regs into a storage array * @regs: Pointer to pt_regs to examine * @store: Pointer to storage array * @size: Size of the storage array
