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SubjectRe: [PATCH 4.19] rcu/tree: Mark functions as notrace
On Wed, 13 Jul 2022 18:20:09 +0800
Zheng Yejian <zhengyejian1@huawei.com> wrote:

> This patch and problem analysis is based on v4.19 LTS, but v5.4 LTS
> and below seem to be involved.
>
> Hulk Robot reports a softlockup problem, see following logs:
> [ 41.463870] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [ksoftirqd/0:9]

This detects something that is spinning with preemption disabled but
interrupts enabled.

> Look into above call stack, there is a recursive call in
> 'ftrace_graph_call', and the direct cause of above recursion is that
> 'rcu_dynticks_curr_cpu_in_eqs' is traced, see following snippet:
> __read_once_size_nocheck.constprop.0
> ftrace_graph_call <-- 1. first call
> ......
> rcu_dynticks_curr_cpu_in_eqs
> ftrace_graph_call <-- 2. recursive call here!!!

This is not the bug. That code can handle a recursion:

ftrace_graph_call is assembly that is converted to call

void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
unsigned long frame_pointer)
{
[..]

bit = ftrace_test_recursion_trylock(ip, *parent);
if (bit < 0)
return;

This will stop the code as "bit" will be < 0 on the second call to
ftrace_graph_call. If it was a real recursion issue, it would crash the
machine when the recursion runs out of stack space.

>
> Comparing with mainline kernel, commit ff5c4f5cad33 ("rcu/tree:
> Mark the idle relevant functions noinstr") mark related functions as
> 'noinstr' which implies notrace, noinline and sticks things in the
> .noinstr.text section.
> Link: https://lore.kernel.org/all/20200416114706.625340212@infradead.org/
>
> But we cannot directly backport that commit, because there seems to be
> many prepatches. Instead, marking the functions as 'notrace' where it is
> 'noinstr' in that commit and mark 'rcu_dynticks_curr_cpu_in_eqs' as
> inline look like it resolves the problem.

That will not fix your problem.

>
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>

Can you reproduce this consistently without this patch, and then not so
with this patch?

Or are you just assuming that this fixes a bug because you observed a
recursion?

Please explain to me why this would cause the hang?

-- Steve

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