On Tue, Mar 4, 2014 at 12:14 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > On Tue, 4 Mar 2014 14:35:51 -0500 > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > >> On Tue, Mar 04, 2014 at 02:10:49PM -0500, Jeff Layton wrote: >> > My expectation is that programs shouldn't mix classic and file-private >> > locks, but Glenn Skinner pointed out to me that that may occur at times >> > even if the programmer isn't aware. >> > >> > Suppose we have a program that uses file-private locks. That program >> > then links in a library that uses classic POSIX locks. If those locks >> > end up conflicting and one is using blocking locks, then the program >> > could end up deadlocked. >> > >> > Try to catch this situation in posix_locks_deadlock by looking for the >> > case where the blocking lock was set by the same process but has a >> > different type, and have the kernel return EDEADLK if that occurs. >> > >> > This check is not perfect. You could (in principle) have a threaded >> > process that is using classic locks in one thread and file-private locks >> > in another. That's not necessarily a deadlockable situation but this >> > check would cause an EDEADLK return in that case. >> > >> > By the same token, you could also have a file-private lock that was >> > inherited across a fork(). If the inheriting process ends up blocking on >> > that while trying to set a classic POSIX lock then this check would miss >> > it and the program would deadlock. >> >> If the caller's not prepared for the library to use classic posix locks, >> then it's not going to know how to recover from this EDEADLCK either, is >> it? >> > > Well, callers should be aware of that if we take this change. The > semantics aren't yet set in stone... > >> I guess I don't understand how this helps anyone. >> >> Has it ever made sense for a library function and its caller to both use >> classic posix locking on the same file without any coordination? >> > > Not really, but that doesn't mean that it isn't done... ;) > >> Besides the first-close problem there's the problem that locks merge, so >> for example you can't hold your own lock across a call to a function >> that grabs and drops a lock on the same file. >> > > It depends, but you're basically correct... > > It's likely that if the above situation occurred with a program using > classic locks, then those locks were silently lost at times. It's also > plausible that when it occurs that no one is aware of it due to the way > POSIX locks work. > > If the program switched to using file-private locks and the library > stays using classic locks (or vice versa), you then potentially trade > that silent loss of locks for a deadlock (since classic and > file-private locks always conflict). > > So, the idea would be to try to catch that situation explicitly and > return a hard error instead of deadlocking. Unfortunately, it's a > little tough to do that in all cases so all this does is try to catch a > subset of them. > > Will it be helpful in the long run? I'm not sure. It seems unlikely to > harm legit use cases though, and might catch some problematic > situations. I can drop this if that's the consensus however. I don't think I like it except in the case where there are no threads (number of tasks sharing the fd table is 1) and where the struct file only has one fd. Otherwise I think it can have false positives. Or am I missing something? --Andy -- 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
