linux-kernel - Re: [PATCH] kallsyms: Fix sleeping function called from invalid context when CONFIG_KALLSYMS

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Message-ID: <Y6GWInExu2m48K/C@alley>
Date:   Tue, 20 Dec 2022 12:01:54 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH] kallsyms: Fix sleeping function called from invalid
 context when CONFIG_KALLSYMS_SELFTEST=y

On Tue 2022-12-20 08:15:40, Christophe Leroy wrote:
> 
> 
> Le 20/12/2022 à 07:39, Zhen Lei a écrit :
> > [T58] BUG: sleeping function called from invalid context at kernel/kallsyms.c:305
> > [T58] in_atomic(): 0, irqs_disabled(): 128, non_block: 0, pid: 58, name: kallsyms_test
> > [T58] preempt_count: 0, expected: 0
> > [T58] RCU nest depth: 0, expected: 0
> > [T58] no locks held by kallsyms_test/58.
> > [T58] irq event stamp: 18899904
> > [T58] hardirqs last enabled at (18899903): finish_task_switch.isra.0 (core.c:?)
> > [T58] hardirqs last disabled at (18899904): test_perf_kallsyms_on_each_symbol (kallsyms_selftest.c:?)
> > [T58] softirqs last enabled at (18899886): __do_softirq (??:?)
> > [T58] softirqs last disabled at (18899879): ____do_softirq (irq.c:?)
> > [T58] CPU: 0 PID: 58 Comm: kallsyms_test Tainted: G T  6.1.0-next-20221215 #2
> > [T58] Hardware name: linux,dummy-virt (DT)
> > [T58] Call trace:
> > [T58] dump_backtrace (??:?)
> > [T58] show_stack (??:?)
> > [T58] dump_stack_lvl (??:?)
> > [T58] dump_stack (??:?)
> > [T58] __might_resched (??:?)
> > [T58] kallsyms_on_each_symbol (??:?)
> > [T58] test_perf_kallsyms_on_each_symbol (kallsyms_selftest.c:?)
> > [T58] test_entry (kallsyms_selftest.c:?)
> > [T58] kthread (kthread.c:?)
> > [T58] ret_from_fork (??:?)
> > [T58] kallsyms_selftest: kallsyms_on_each_symbol() traverse all: 5744310840 ns
> > [T58] kallsyms_selftest: kallsyms_on_each_match_symbol() traverse all: 1164580 ns
> > [T58] kallsyms_selftest: finish
> > 
> > Functions kallsyms_on_each_symbol() and kallsyms_on_each_match_symbol()
> > call the user-registered hook function for each symbol that meets the
> > requirements. Because it is uncertain how long that hook function will
> > execute, they call cond_resched() to avoid consuming CPU resources for a
> > long time. However, irqs need to be disabled during the performance test
> > to ensure the accuracy of test data. Because the performance test hook is
> > very clear, very simple function, let's do not call cond_resched() when
> > CONFIG_KALLSYMS_SELFTEST=y.
> 
> I don't think it is appropriate to change the behaviour of a core 
> function based on whether a compile time option related to tests is 
> selected or not, because you will change the behaviour for all users, 
> not only for the tests.

I agree. This is very bad idea. It would change the behavior for
the entire system.

> If the problem is that IRQs are disabled, maybe the solution is
> 
> 	if (!irqs_disabled())
> 		cond_resched();
> 
> Or try to disable the call to cond_resched() in a way or another during 
> the run of selftests.

If I get it correctly, the problem is this code in kernel/kallsyms_selftest.c:

static int lookup_name(void *data, const char *name, struct module *mod, unsigned long addr)
{
[...]
	local_irq_save(flags);
	t0 = sched_clock();
	(void)kallsyms_lookup_name(name);
	t1 = sched_clock();
	local_irq_restore(flags);
[...]

and IRQs are disabled to measure the time spent in this function
without interruption and rescheduling.

I am sure that there are better ways how to measure the time.
Even the "time" command in userspace is able to show time how much CPU
time a command used.

I am not familiar with it. But task_cputime() in
kernel/sched/cputime.c looks promising. And there must be
the interface how the user space get this information.
Some is available via /proc/<PID>/... I am not sure
if there is a syscall.

Best Regards,
Petr