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

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Date:   Wed, 12 Dec 2018 15:39:01 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH] debugobjects: Move printk out of db lock critical
 sections

On Wed, 12 Dec 2018 17:28:14 -0500 Waiman Long <longman@...hat.com> 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.

Patch looks sensible, but I have a nit about the variable names.

> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -375,6 +375,8 @@ static void debug_object_is_on_stack(void *addr, int onstack)
>  	struct debug_bucket *db;
>  	struct debug_obj *obj;
>  	unsigned long flags;
> +	bool debug_printobj = false;

"debug_printobject" would be better, but this code already intermingles
"obj" and "object".

> +	bool debug_chkstack = false;

Not so good.  Is it debug_chkstack or debug_checkstk or ...

This file uses "check" consistently so let's not depart from that? 
Linux style is to avoid these tricky little abbreviations and to use
full words.  

ie, debug_checkstack, please.  Better would be debug_check_stack.  Or
simply check_stack: the "debug" doesn't add anything useful.