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Date:   Tue, 26 Jan 2021 14:59:17 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc:     Petr Mladek <pmladek@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] Fix BUG: Invalid wait context in hrtimer_interrupt()
Message-ID: <20210126145917.r64vzibgpiwyuake@e107158-lin>
References: <20210123233741.3614408-1-qais.yousef@arm.com>
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On 01/26/21 13:46, Sergey Senozhatsky wrote:
> On (21/01/23 23:37), Qais Yousef wrote:
> > 
> > I hit a pr_warn() inside hrtimer_interrupt() which lead to a BUG: Invalid wait
> > context splat.
> > 
> > The problem wasn't reproducible but I think the cause is obvious, printk can't
> > be called from interrupt context.
> > 
> > AFAICU printk_deferred() is safe from NMI, so I assumed it is safe to be called
> > from hrtimer_interrupt() too. Adding a pr_warn_once() inside
> > hrtimer_interrupt() in a location where it is always hit produces the BUG
> > splat. Replacing it with pr_warn_deferred_once() generates the printk warning
> > without any splat.
> 
> Could you please post the lockdep splat?

Sorry I should have done that. I thought printk() in interrupt is not allowed
and obvious.

I reported it here: https://lore.kernel.org/lkml/20210114124512.mg3vexudfmoadef5@e107158-lin/

	# [67628.388606] hrtimer: interrupt took 304720 ns
	[67628.393546]
	[67628.393550] =============================
	[67628.393554] [ BUG: Invalid wait context ]
	[67628.393557] 5.11.0-rc3-00019-g86be331946f7 #37 Not tainted
	[67628.393560] -----------------------------
	[67628.393563] sugov:0/192 is trying to lock:
	[67628.393566] ffff000800b1d898 (&port_lock_key){-.-.}-{3:3}, at: pl011_console_write+0x138/0x218
	[67628.393581] other info that might help us debug this:
	[67628.393584] context-{2:2}
	[67628.393586] 4 locks held by sugov:0/192:
	[67628.393589]  #0: ffff0008059cb720 (&sg_policy->work_lock){+.+.}-{4:4}, at: sugov_work+0x58/0x88
	[67628.393603]  #1: ffff800015446f20 (prepare_lock){+.+.}-{4:4}, at: clk_prepare_lock+0x34/0xb0
	[67628.393618]  #2: ffff8000152aaa60 (console_lock){+.+.}-{0:0}, at: vprintk_emit+0x12c/0x310
	[67628.393632]  #3: ffff8000152aab88 (console_owner){-.-.}-{0:0}, at: console_unlock+0x190/0x6d8
	[67628.393646] stack backtrace:
	[67628.393649] CPU: 0 PID: 192 Comm: sugov:0 Not tainted 5.11.0-rc3-00019-g86be331946f7 #37
	[67628.393653] Hardware name: ARM Juno development board (r2) (DT)
	[67628.393656] Call trace:
	[67628.393659]  dump_backtrace+0x0/0x1b0
	[67628.393661]  show_stack+0x20/0x70
	[67628.393664]  dump_stack+0xf8/0x168
	[67628.393666]  __lock_acquire+0x964/0x1c88
	[67628.393669]  lock_acquire+0x26c/0x500
	[67628.393671]  _raw_spin_lock+0x64/0x88
	[67628.393674]  pl011_console_write+0x138/0x218
	[67628.393677]  console_unlock+0x2c4/0x6d8
	[67628.393680]  vprintk_emit+0x134/0x310
	[67628.393682]  vprintk_default+0x40/0x50
	[67628.393685]  vprintk_func+0xfc/0x2b0
	[67628.393687]  printk+0x68/0x90
	[67628.393690]  hrtimer_interrupt+0x24c/0x250
	[67628.393693]  arch_timer_handler_phys+0x3c/0x50
	[67628.393695]  handle_percpu_devid_irq+0xd8/0x460
	[67628.393698]  generic_handle_irq+0x38/0x50
	[67628.393701]  __handle_domain_irq+0x6c/0xc8
	[67628.393704]  gic_handle_irq+0xb0/0xf0
	[67628.393706]  el1_irq+0xb4/0x180
	[67628.393709]  _raw_spin_unlock_irqrestore+0x58/0xa8
	[67628.393712]  hrtimer_start_range_ns+0x1b0/0x420
	[67628.393715]  msg_submit+0x100/0x108
	[67628.393717]  mbox_send_message+0x84/0x128
	[67628.393720]  scpi_send_message+0x11c/0x2a8
	[67628.393723]  scpi_dvfs_set_idx+0x48/0x70
	[67628.393726]  scpi_dvfs_set_rate+0x60/0x78
	[67628.393728]  clk_change_rate+0x184/0x8a8
	[67628.393731]  clk_core_set_rate_nolock+0x1c0/0x280
	[67628.393734]  clk_set_rate+0x40/0x98
	[67628.393736]  scpi_cpufreq_set_target+0x40/0x68
	[67628.393739]  __cpufreq_driver_target+0x1a0/0x5d0

> Why is it invalid? Is this... -rt kernel, perhaps?

No I was running on Linus master at the time.

AFAIU printk will hold the console lock which could sleep in interrupt context.

Did I miss something?

Thanks

--
Qais Yousef