linux-kernel - Re: [PATCH] debugobjects: Move printk out of db lock critical sections

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Message-ID: <9cb1526a-e82f-dfa7-4b3c-5ea911a8d476@arista.com>
Date:   Thu, 13 Dec 2018 04:35:04 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Arnd Bergmann <arnd@...db.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] debugobjects: Move printk out of db lock critical
 sections

Hi Waiman,

On 12/12/18 10:28 PM, Waiman Long wrote:
> The db->lock is a raw spinlock and so the lock hold time is supposed
> to be short. This will not be the case when printk() is being involved
> in some of the critical sections. In order to avoid the long hold time,
> in case some messages need to be printed, the debug_object_is_on_stack()
> and debug_print_object() calls are now moved out of those critical
> sections.
> 
> Holding the db->lock while calling printk() may lead to deadlock if
> printk() somehow requires the allocation/freeing of debug object that
> happens to be in the same hash bucket or a circular lock dependency
> warning from lockdep as reported in https://lkml.org/lkml/2018/12/11/143.
> 
> [   87.209665] WARNING: possible circular locking dependency detected
> [   87.210547] 4.20.0-rc4-00057-gc96cf92 #1 Tainted: G        W
> [   87.211449] ------------------------------------------------------
> [   87.212405] getty/519 is trying to acquire lock:
> [   87.213074] (____ptrval____) (&obj_hash[i].lock){-.-.}, at: debug_check_no_obj_freed+0xb4/0x302
> [   87.214343]
> [   87.214343] but task is already holding lock:
> [   87.215174] (____ptrval____) (&port_lock_key){-.-.}, at: uart_shutdown+0x3a3/0x4e2
> [   87.216260]
> [   87.216260] which lock already depends on the new lock.
> 
> This patch was also found to be able to fix a boot hanging problem
> when the initramfs image was switched on after a debugobjects splat
> from the EFI code.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---

I've tried to review it and found minor issues like missed
debug_object_is_on_stack() for initializing already active object.

But than I come to opinion that it's just generally unsafe:
debug_obj life-time is protected by bucket's spin_lock.
Check the conditions when free_object() is being called.

So, I resulted by finishing my stack-copy version which I've started
yesterday on test robot report when I didn't know about your patch.

I've sent it now concurrently.

Thanks,
          Dima