linux-kernel - Re: possible deadlock in lru_add_drain

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Date:   Mon, 30 Oct 2017 16:10:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot 
        <bot+e7353c7141ff7cbb718e4c888a14fa92de41ebaa@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        jglisse@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, shli@...com, syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, ying.huang@...el.com,
        kernel-team@....com
Subject: Re: possible deadlock in lru_add_drain_all

On Mon, Oct 30, 2017 at 07:09:21PM +0900, Byungchul Park wrote:
> On Mon, Oct 30, 2017 at 09:22:03AM +0100, Michal Hocko wrote:
> > [Cc Byungchul. The original full report is
> > http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@google.com]
> > 
> > Could you have a look please? This smells like a false positive to me.
> 
> +cc peterz@...radead.org
> 
> Hello,
> 
> IMHO, the false positive was caused by the lockdep_map of 'cpuhp_state'
> which couldn't distinguish between cpu-up and cpu-down.
> 
> And it was solved with the following commit by Peter and Thomas:
> 
> 5f4b55e10645b7371322c800a5ec745cab487a6c
> smp/hotplug: Differentiate the AP-work lockdep class between up and down
> 
> Therefore, we can avoid the false positive on later than the commit.
> 
> Peter and Thomas, could you confirm it?

I can indeed confirm it's running old code; cpuhp_state is no more.

However, that splat translates like:

	__cpuhp_setup_state()
#0	  cpus_read_lock()
	  __cpuhp_setup_state_cpuslocked()
#1	    mutex_lock(&cpuhp_state_mutex)



	__cpuhp_state_add_instance()
#2	  mutex_lock(&cpuhp_state_mutex)
	  cpuhp_issue_call()
	    cpuhp_invoke_ap_callback()
#3	      wait_for_completion()

						msr_device_create()
						  ...
#4						    filename_create()
#3						complete()



	do_splice()
#4	  file_start_write()
	  do_splice_from()
	    iter_file_splice_write()
#5	      pipe_lock()
	      vfs_iter_write()
	        ...
#6		  inode_lock()



	sys_fcntl()
	  do_fcntl()
	    shmem_fcntl()
#5	      inode_lock()
	      shmem_wait_for_pins()
	        if (!scan)
		  lru_add_drain_all()
#0		    cpus_read_lock()



Which is an actual real deadlock, there is no mixing of up and down.