On Wed, 20 Nov 2013 14:50:32 -0500
"J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> On Wed, Nov 20, 2013 at 11:45:04AM -0500, Jeff Layton wrote:
> > Due to some unfortunate history, POSIX locks have very strange and
> > unhelpful semantics. The thing that usually catches people by surprise
> > is that they are dropped whenever the process closes any file descriptor
> > associated with the inode.
> >
> > This is extremely problematic for people developing file servers that
> > need to implement byte-range locks. Developers often need a "lock
> > management" facility to ensure that file descriptors are not closed
> > until all of the locks associated with the inode are finished.
> >
> > This patchset adds a new type of lock that attempts to address this
> > issue. These locks work just like "normal" POSIX read/write locks, but
> > have semantics that are more like BSD locks with respect to inheritance
> > and behavior on close.
> >
> > This is implemented primarily by changing how fl_owner field is set for
> > these locks. Instead of having them owned by the files_struct of the
> > process, they are instead owned by the filp on which they were acquired.
> > Thus, they are inherited across fork() and are only released when the
> > last reference to a filp is put.
> >
> > These new semantics prevent them from being merged with "classic" POSIX
> > locks, even if they are acquired by the same process. These locks will
> > also conflict with "classic" POSIX locks even if they are acquired by
> > the same process or on the same file descriptor.
> >
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > fs/locks.c | 22 +++++++++++++++++++++-
> > include/uapi/asm-generic/fcntl.h | 15 +++++++++++++++
> > 2 files changed, 36 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/locks.c b/fs/locks.c
> > index 86cafc3..3b278a6 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -348,6 +348,26 @@ static int posix_assign_type(struct file_lock *fl, long type)
> > {
> > int err;
> >
> > + /*
> > + * FL_FILP_PRIVATE locks are "owned" by the filp upon which they were
> > + * acquired, regardless of what task is dealing with them. Set the
> > + * fl_owner appropriately.
> > + */
> > + switch (type) {
> > + case F_RDLCKP:
> > + type = F_RDLCK;
> > + fl->fl_owner = (fl_owner_t)fl->fl_file;
> > + break;
> > + case F_WRLCKP:
> > + type = F_WRLCK;
> > + fl->fl_owner = (fl_owner_t)fl->fl_file;
> > + break;
> > + case F_UNLCKP:
> > + type = F_UNLCK;
> > + fl->fl_owner = (fl_owner_t)fl->fl_file;
> > + break;
> > + }
> > +
>
> After this fl_owner gets set to current->files in
> flock{64}_to_posix_lock and then reset here. That seems like a trap for
> the unwary reader.
>
> Could you do something like rename this flock_to_posix_lock_common and
> move all the 32/64-bit-independent initialization here?
>
> Looks like there's way more duplication than necessary between those two
> cases.
>
Good point. I'll look at doing some more cleanup in that direction on
the next set.
> (Also, why do we have an fl_owner_t instead of using a void?)
>
Another good point. As far as I can tell nothing uses fl_owner for
anything but a generic ownership "cookie". The pointer is never
actually dereferenced to get back to a files_struct, so a void pointer
(or even an unsigned long) would make more sense there.
I'll look at that before sending out the next version of this.
Thanks,
>
> > err = assign_type(fl, type);
> > if (err)
> > return err;
> > @@ -2225,7 +2245,7 @@ void locks_remove_filp(struct file *filp)
> >
> > while ((fl = *before) != NULL) {
> > if (fl->fl_file == filp) {
> > - if (IS_FLOCK(fl)) {
> > + if (IS_FLOCK(fl) || IS_POSIX(fl)) {
> > locks_delete_lock(before);
> > continue;
> > }
> > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> > index 95e46c8..6b7b68a 100644
> > --- a/include/uapi/asm-generic/fcntl.h
> > +++ b/include/uapi/asm-generic/fcntl.h
> > @@ -151,6 +151,21 @@ struct f_owner_ex {
> > #define F_UNLCK 2
> > #endif
> >
> > +/*
> > + * fd "private" POSIX locks.
> > + *
> > + * Usually POSIX locks held by a process are released on *any* close and are
> > + * not inherited across a fork().
> > + *
> > + * These lock types will conflict with normal POSIX locks, but are "owned"
> > + * by the fd, not the process. This means that they are inherited across
> > + * fork() like BSD (flock) locks, and they are only closed when the last
> > + * reference to the the filp against which were acquired is closed.
> > + */
> > +#define F_RDLCKP 5
> > +#define F_WRLCKP 6
> > +#define F_UNLCKP 7
> > +
> > /* for old implementation of bsd flock () */
> > #ifndef F_EXLCK
> > #define F_EXLCK 4 /* or 3 */
> > --
> > 1.8.3.1
> >
> > --
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--
Jeff Layton <jlayton@xxxxxxxxxx>
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