On Tue, 2007-09-18 at 12:52 -0400, J. Bruce Fields wrote: > On Tue, Sep 18, 2007 at 12:14:55PM -0400, Trond Myklebust wrote: > > Note also that strictly speaking, we're not even compliant with the > > System V behaviour on read() and write(). See: > > > > http://www.unix.org.ua/orelly/networking_2ndEd/nfs/ch11_01.htm > > and > > > > http://docs.sun.com/app/docs/doc/801-6736/6i13fom0a?l=en&a=view&q=mandatory+lock > > > > According to these docs, we should be wrapping each and every read() and > > write() syscall with a mandatory lock. The fact that we're not, and yet > > still not seeing any complaints just goes to show how few people are > > actually using and relying on this... > > 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 :-) Trond - 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/
