2018年11月21日(水) 13:54 Al Viro <v...@zeniv.linux.org.uk>:
>
> On Wed, Nov 21, 2018 at 11:43:56AM +0900, Eiichi Tsukata wrote:
> > Some file systems (including ext4, xfs, ramfs ...) have the following
> > problem as I've described in the commit message of the 1/4 patch.
> >
> > The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
> > removed almost all locks in llseek() including SEEK_END. It based on the
> > idea that write() updates size atomically. But in fact, write() can be
> > divided into two or more parts in generic_perform_write() when pos
> > straddles over the PAGE_SIZE, which results in updating size multiple
> > times in one write(). It means that llseek() can see the size being
> > updated during write().
>
> And? Who has ever promised anything that insane? write(2) can take an
> arbitrary
> amount of time; another process doing lseek() on independently opened
> descriptor
> is *not* going to wait for that (e.g. page-in of the buffer being written,
> which
> just happens to be mmapped from a file on NFS over RFC1149 link).
Thanks.
The lock I added in NFS was nothing but slow down lseek() because a file size is
updated atomically. Even `spin_lock(&inode->i_lock)` is unnecessary.
I'll fix the commit message which only refers to specific local file
systems that use
generic_perform_write() and remove unnecessary locks in some
distributed file systems
(e.g. nfs, cifs, or more) by replacing generic_file_llseek() with
generic_file_llseek_unlocked()
so that `tail` don't have to wait for avian carriers.
