linux-kernel - Re: [PATCH 2/3] fd/locks: allow get the lock owner by F_OFD

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Date:   Wed, 21 Jun 2023 07:54:13 +0500
From:   stsp <stsp2@...dex.ru>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org,
        Chuck Lever <chuck.lever@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] fd/locks: allow get the lock owner by F_OFD_GETLK

20.06.2023 22:05, Matthew Wilcox пишет:
>> Does this mean, by any chance, that the
>> recipient actually owns an fd before
>> recvmsg() is done?
> no, it's not in their fd table.  they don't own it.
OK, thanks for showing this pathological
case. Let me just note that this changes
nothing at all. :)

The important thing to note here is that
any lock query is race-prone: locks can
come and go at any time. So if you need
some sequence of operations, you need
to employ some global locking for that.
I use flock(LOCK_EX) on the same fd, before
doing F_OFD_GETLK, and I do flock(LOCK_UN)
only when the entire sequence of operations
is completed. And I do the same on an
F_OFD_SETLK's side to guarantee the
atomicity. You can't do it otherwise,
it would be race-prone.

So given the above, the only thing we
need for l_pid consistency is for the
"donor" process to put LOCK_EX on an
fd before doing SCM_RIGHTS, and the
recipient should do LOCK_UN. Then
the other side, which also uses LOCK_EX,
will never see the owner-less state.
And as for the kernel's POV, l_pid should
be set to -1 only when there is no owner,
like in an example you mentioned.