Messages in this thread |  | | Date | Tue, 17 Feb 2015 14:08:44 -0500 | From | "J. Bruce Fields" <> | Subject | Re: [GIT PULL] please pull file-locking related changes for v3.20 |
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On Mon, Feb 16, 2015 at 11:24:03AM -0800, Linus Torvalds wrote: > On Mon, Feb 16, 2015 at 10:46 AM, Linus Torvalds > <torvalds@linux-foundation.org> 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.
--b.
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