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From: "J. Bruce Fields" <bfields@fieldses.org>
To: NeilBrown <neilb@suse.com>
Cc: Jeff Layton <jlayton@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Martin Wilck <mwilck@suse.de>,
	linux-fsdevel@vger.kernel.org,
	Frank Filz <ffilzlnx@mindspring.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups
Date: Thu, 9 Aug 2018 13:32:45 -0400
Message-ID: <20180809173245.GM23873@fieldses.org> (raw)
In-Reply-To: <153378012255.1220.6754153662007899557.stgit@noble>

I think there's also a problem with multiple tasks sharing the same
lock owner.

So, all locks are exclusive locks for the same range.  We have four
tasks.  Tasks 1 and 4 share the same owner, the others' owners are
distinct.

	- Task 1 gets a lock.
	- Task 2 gets a conflicting lock.
	- Task 3 gets another conflicting lock.  So now we the tree is
		3->2->1.
	- Task 1's lock is released.
	- Before task 2 is scheduled, task 4 acquires a new lock.
	- Task 2 waits on task 4's lock, we now have
		3->2->4.

Task 3 shouldn't be waiting--the lock it's requesting has the same owner
as the lock task 4 holds--but we fail to wake up task 3.

--b.

  parent reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-09  2:04 NeilBrown
2018-08-09  2:04 ` [PATCH 1/5] fs/locks: rename some lists and pointers NeilBrown
2018-08-09  2:04 ` [PATCH 2/5] fs/locks: allow a lock request to block other requests NeilBrown
2018-08-09  2:04 ` [PATCH 3/5] fs/locks: change all *_conflict() functions to return a new enum NeilBrown
2018-08-09 11:09   ` Jeff Layton
2018-08-09 13:09   ` J. Bruce Fields
2018-08-09 23:40     ` NeilBrown
2018-08-10  0:56       ` J. Bruce Fields
2018-08-09  2:04 ` [PATCH 4/5] fs/locks: split out __locks_wake_one() NeilBrown
2018-08-09  2:04 ` [PATCH 5/5] fs/locks: create a tree of dependent requests NeilBrown
2018-08-09 11:17   ` Jeff Layton
2018-08-09 23:25     ` NeilBrown
2018-08-09 14:13   ` J. Bruce Fields
2018-08-09 22:19     ` NeilBrown
2018-08-10  0:36       ` J. Bruce Fields
2018-08-09 17:32 ` J. Bruce Fields [this message]
2018-08-09 22:12   ` [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups NeilBrown
2018-08-10  0:29     ` J. Bruce Fields
2018-08-10  1:50       ` NeilBrown
2018-08-10  2:52         ` J. Bruce Fields
2018-08-10  3:17           ` NeilBrown
2018-08-10 15:47             ` J. Bruce Fields
2018-08-11 11:56               ` Jeff Layton
2018-08-11 12:35                 ` J. Bruce Fields
2018-08-11 11:51       ` Jeff Layton
2018-08-11 12:21         ` J. Bruce Fields
2018-08-11 13:15           ` Jeff Layton

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