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From: David Hildenbrand <david@redhat.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	Baoquan He <bhe@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vivek Goyal <vgoyal@redhat.com>, Dave Young <dyoung@redhat.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v1] proc/vmcore: fix false positive lockdep warning
Date: Wed, 19 Jan 2022 16:19:16 +0100	[thread overview]
Message-ID: <035dc6b1-28cb-0563-c712-bf57ebbc91e8@redhat.com> (raw)
In-Reply-To: <ec19d477-a75c-1e5a-1c02-f62c8565f48d@redhat.com>

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


WARNING: multiple messages have this Message-ID (diff)
From: David Hildenbrand <david@redhat.com>
To: kexec@lists.infradead.org
Subject: [PATCH v1] proc/vmcore: fix false positive lockdep warning
Date: Wed, 19 Jan 2022 16:19:16 +0100	[thread overview]
Message-ID: <035dc6b1-28cb-0563-c712-bf57ebbc91e8@redhat.com> (raw)
In-Reply-To: <ec19d477-a75c-1e5a-1c02-f62c8565f48d@redhat.com>

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



  reply	other threads:[~2022-01-19 15:19 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-19 11:37 [PATCH v1] proc/vmcore: fix false positive lockdep warning David Hildenbrand
2022-01-19 11:37 ` David Hildenbrand
2022-01-19 15:08 ` Boqun Feng
2022-01-19 15:08   ` Boqun Feng
2022-01-19 15:15   ` David Hildenbrand
2022-01-19 15:15     ` David Hildenbrand
2022-01-19 15:19     ` David Hildenbrand [this message]
2022-01-19 15:19       ` David Hildenbrand

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