linux-kernel - Re: [PATCH v5 13/14] locks: skip deadlock detection on FL_FILE

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Date:	Tue, 14 Jan 2014 13:26:26 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Frank Filz <ffilzlnx@...dspring.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	Jeff Layton <jlayton@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	nfs-ganesha-devel@...ts.sourceforge.net,
	samba-technical@...ts.samba.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Richard Hipp <drh@...ci.com>
Subject: Re: [PATCH v5 13/14] locks: skip deadlock detection on FL_FILE_PVT locks

[grr, gmail -- I didn't actually intend to send that.]

On Tue, Jan 14, 2014 at 1:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Jan 14, 2014 at 1:19 PM, Frank Filz <ffilzlnx@...dspring.com> wrote:
>>>       process 2 requests a write lock, gets -EDEADLK, unlocks and
>>>       requests a new read lock.  That request succeeds because there
>>>       is no conflicting lock.  (Note the lock manager had no
>>>       opportunity to upgrade 1's lock here thanks to the conflict with
>>>       3's lock.)
>>
>> As I understand write lock priority, process 2 requesting a new read lock
>> would block, once there is a write lock waiter, no further read locks would
>> be granted that would conflict with that waiting write lock.
>
> ...which reminds me -- if anyone implements writer priority, please
> make it optional (either w/ a writer-priority-ignoring read lock or a
> non-priority-granting write lock).  I have an application for which
> writer priority would be really annoying.
>
> Even better: Have read-lock-and-wait-for-pending-writers be an explicit new operation.
>
> (Writer priority a

Writer priority can introduce new deadlocks.  Suppose that a reader
(holding a read lock) starts a subprocess that takes a new read lock
and waits for that subprocess.  Throw an unrelated process in that
tries to take a write lock and you have an instant deadlock.

--Andy
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