On Tue, Sep 18, 2007 at 12:54:56PM -0400, Trond Myklebust wrote: > On Tue, 2007-09-18 at 12:52 -0400, J. Bruce Fields wrote: > > So currently there's nothing to prevent this: > > > > - write passes locks_mandatory_area() checks > > - get mandatory lock > > - read old data > > - write updates file data > > - read new data > > > > You can see the data change even while you hold a mandatory lock that > > should exclude writes. > > > > Similarly you might think that an application could prevent anyone from > > seeing the intermediate state of a file while it performs a series of > > writes under an exclusive mandatory lock, but actually there's nothing > > to stop a read in progress from racing with acquisition of the lock. > > > > Unless I'm missing something, that makes our mandatory lock > > implementation pretty pointless. I wish we could either fix it or just > > ditch it, but I suppose either option would be unpopular. > > It gets even better when you throw mmap() into the mix :-)
Hm. Documentation/mandatory.txt claims that it mandatory locks and mmap() with MAP_SHARED exclude each other, but I can't see where that's enfoced. That file doesn't make any mention of the above race. So for now I think someone should update that file and fcntl(2) to mention these problems and to recommend rather strongly against using mandatory locking. --b. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
