Re: Regression in v4.19.106 breaking waking up of readers of /proc/kmsg and /dev/kmsg

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Feb 28, 2020 at 02:02:17PM +0100, Petr Mladek wrote:
> On Fri 2020-02-28 12:32:14, Petr Mladek wrote:
> > On Fri 2020-02-28 11:58:36, Greg Kroah-Hartman wrote:
> > > On Fri, Feb 28, 2020 at 11:04:16AM +0100, Petr Mladek wrote:
> > > > On Fri 2020-02-28 12:13:06, Sergey Senozhatsky wrote:
> > > > > Cc-ing Petr, Steven, John
> > > > > 
> > > > > https://lore.kernel.org/lkml/e9358218-98c9-2866-8f40-5955d093dc1b@camlintechnologies.com
> > > > > 
> > > > > On (20/02/27 14:08), Lech Perczak wrote:
> > > > > > W dniu 27.02.2020 o 13:39, Lech Perczak pisze:
> > > > > > > W dniu 27.02.2020 o 13:36, Greg Kroah-Hartman pisze:
> > > > > > >> On Thu, Feb 27, 2020 at 11:09:49AM +0000, Lech Perczak wrote:
> > > > > > >>> Hello,
> > > > > > >>>
> > > > > > >>> After upgrading kernel on our boards from v4.19.105 to v4.19.106 we found out that syslog fails to read the messages after ones read initially after opening /proc/kmsg just after booting.
> > > > > > >>> I also found out, that output of 'dmesg --follow' also doesn't react on new printks appearing for whatever reason - to read new messages, reopening /proc/kmsg or /dev/kmsg was needed.
> > > > > > >>> I bisected this down to commit 15341b1dd409749fa5625e4b632013b6ba81609b ("char/random: silence a lockdep splat with printk()"), and reverting it on top of v4.19.106 restored correct behaviour.
> > > > > > >> That is really really odd.
> > > > > > > Very odd it is indeed.
> > > > > > >>> My test scenario for bisecting was:
> > > > > > >>> 1. run 'dmesg --follow' as root
> > > > > > >>> 2. run 'echo t > /proc/sysrq-trigger'
> > > > > > >>> 3. If trace appears in dmesg output -> good, otherwise, bad. If trace doesn't appear in output of 'dmesg --follow', re-running it will show the trace.
> > > > > > >>>
> > > > 
> > > > I have reproduced the problem with a kernel based on v4.19.106
> > > > and I see the following in the log:
> > > > 
> > > > [    0.028250] clocksource: refined-jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645519600211568 ns
> > > > [    0.028263] random: get_random_bytes called from start_kernel+0x9e/0x4f6 with crng_init=0
> > > > [    0.028268] setup_percpu: NR_CPUS:8192 nr_cpumask_bits:4 nr_cpu_ids:4 nr_node_ids:1
> > > > [    0.028407] percpu: Embedded 44 pages/cpu s142216 r8192 d29816 u524288
> > > > [    0.028411] pcpu-alloc: s142216 r8192 d29816 u524288 alloc=1*2097152
> > > > [    0.028412] pcpu-alloc: [0] 0 1 2 3 
> > > > 
> > > > Note that percpu stuff is initialized after printk_deferred(). And the
> > > > deferred console is scheduled by:
> > > > 
> > > > void defer_console_output(void)
> > > > {
> > > > 	preempt_disable();
> > > > 	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> > > > 	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> > > > 	preempt_enable();
> > > > }
> > > > 
> > > > I am afraid that the patch creates some mess via the non-initialized
> > > > per-cpu variable.
> > > > 
> > > > I see that x86 has some support for EARLY_PER_CPU stuff but it seems
> > > > to be arch-specific.
> > > > 
> > > > I do not see a reliable way to detect when per-cpu variables are
> > > > initialized. Adding Tejun and PeterZ into CC if they have any
> > > > idea.
> > > > 
> > > > I suggest to revert the patch until we have some easy and safe solution.
> > > 
> > > Ok, I'll do so, but why is this not an issue in 5.4.y and newer kernels?
> > 
> > Good question. Well, there have been many changes in the random number
> > subsystem initialization recently. My bet is that it is much harder to
> > hit the warning there.
> 
> OK, the problem has gone after I cherry-picked the commit
> d55535232c3dbde9a523 ("random: move rand_initialize() earlier").
> I still see the warning but it is printed much later:
> 
> [    0.051846] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=4
> [    0.054070] NR_IRQS: 524544, nr_irqs: 456, preallocated irqs: 16
> [    0.054281] random: get_random_bytes called from start_kernel+0x308/0x4fe with crng_init=0
> [    0.054430] Console: colour dummy device 80x25
> 
> But I am not sure if it is safe to backport this patch into the old
> stable kernel.
> 
> Anyway, this fix would not be enough. The commit message mentions:
> 
>     Note that this warning may still remain for machines that do not have
>     UEFI RNG support (which initializes the RNG pools during setup_arch()),
>     or for x86 machines without RDRAND (or booting without "random.trust=on"
>     or CONFIG_RANDOM_TRUST_CPU=y).
> 
> 
> So, I would still prefer to _revert_ the commit 15341b1dd409749f
> ("char/random: silence a lockdep splat with printk()"). It calmed
> down lockdep report. The real life danger is dubious. The warning
> is printed early when the system is running on single CPU where
> it could not race.

Yeah, good idea, it's now reverted in my tree and will show up in the
next release.

thanks everyone for finding and working on this.

greg k-h