linux-kernel - Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups

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Message-ID: <20180809173245.GM23873@fieldses.org>
Date:   Thu, 9 Aug 2018 13:32:45 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Martin Wilck <mwilck@...e.de>, linux-fsdevel@...r.kernel.org,
        Frank Filz <ffilzlnx@...dspring.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups

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.