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@xxxxxxxxxxxxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
