linux-kernel - Re: [PATCH v1] proc/vmcore: fix false positive lockdep warning

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Message-ID: <035dc6b1-28cb-0563-c712-bf57ebbc91e8@redhat.com>
Date:   Wed, 19 Jan 2022 16:19:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Baoquan He <bhe@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1] proc/vmcore: fix false positive lockdep warning

On 19.01.22 16:15, David Hildenbrand wrote:
> On 19.01.22 16:08, Boqun Feng wrote:
>> Hi,
>>
>> On Wed, Jan 19, 2022 at 12:37:02PM +0100, David Hildenbrand wrote:
>>> Lockdep complains that we do during mmap of the vmcore:
>>> 	down_write(mmap_lock);
>>> 	down_read(vmcore_cb_rwsem);
>>> And during read of the vmcore:
>>> 	down_read(vmcore_cb_rwsem);
>>> 	down_read(mmap_lock);
>>>
>>> We cannot possibly deadlock when only taking vmcore_cb_rwsem in read
>>> mode, however, it's hard to teach that to lockdep.
>>>
>>
>> Lockdep warned about the above sequences because rw_semaphore is a fair
>> read-write lock, and the following can cause a deadlock:
>>
>> 	TASK 1			TASK 2		TASK 3
>> 	======			======		======
>> 	down_write(mmap_lock);
>> 				down_read(vmcore_cb_rwsem)
>> 						down_write(vmcore_cb_rwsem); // blocked
>> 	down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
>> 				down_read(mmap_lock); // blocked
>> 	
>> IOW, a reader can block another read if there is a writer queued by the
>> second reader and the lock is fair.
>>
>> So there is a deadlock possiblity.
> 
> Task 3 will never take the mmap_lock before doing a
> down_write(vmcore_cb_rwsem).
> 
> How would this happen?

Ah, I get it, nevermind. I'll adjust the patch description.

Thanks!

-- 
Thanks,

David / dhildenb