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SubjectRe: [GIT PULL] please pull file-locking related changes for v3.20
On Mon, Feb 16, 2015 at 11:24:03AM -0800, Linus Torvalds wrote:
> On Mon, Feb 16, 2015 at 10:46 AM, Linus Torvalds
> <> wrote:
> >
> > This code is so broken that my initial reaction is "We need to just
> > revert the crap".
> How the hell is flock_lock_file() supposed to work at all, btw?
> Say we have an existing flock, and now do a new one that conflicts. I
> see what looks like three separate bugs.
> - We go through the first loop, find a lock of another type, and
> delete it in preparation for replacing it
> - we *drop* the lock context spinlock.
> - BUG #1? So now there is no lock at all, and somebody can come in
> and see that unlocked state. Is that really valid?
> - another thread comes in while the first thread dropped the lock
> context lock, and wants to add its own lock. It doesn't see the
> deleted or pending locks, so it just adds it
> - the first thread gets the context spinlock again, and adds the lock
> that replaced the original
> - BUG #2? So now there are *two* locks on the thing, and the next
> time you do an unlock (or when you close the file), it will only
> remove/replace the first one.
> Both of those bugs are due to the whole "drop the lock in the middle",
> which is pretty much always a mistake. BUG#2 could easily explain the
> warning Kirill reports, afaik.
> BUG#3 seems to be independent, and is about somebody replacing an
> existing lock, but the new lock conflicts. Again, the first loop will
> remove the old lock, and then the second loop will see the conflict,
> and return an error (and we may then end up waiting for it for the
> FILE_LOCK_DEFERRED case). Now the original lock is gone. Is that
> really right? That sounds bogus. *Failing* to insert a flock causing
> the old flock to go away?

From flock(2):

Converting a lock (shared to exclusive, or vice versa) is not
guaranteed to be atomic: the existing lock is first removed,
and then a new lock is established. Between these two steps, a
pending lock request by another process may be granted, with
the result that the conversion either blocks, or fails if
LOCK_NB was specified.

I also checked Michael Kerrisk's book quickly and see similar language
plus "... the conversion will fail and the process will lose its
original lock".

I don't have a quick way to check BSD, but it looks to me like this is
the way Linux has always behaved.

I agree that it's weird, but I think it's what we're stuck with.


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