Re: [RFC PATCH 08/12] nfs: convert lock handling to use file_lock_context

Jeff Layton
2014-09-10T19:28:15Z

On Wed, 10 Sep 2014 15:17:34 -0400
"J. Bruce Fields" <bfields@fieldses.org> wrote:

> On Wed, Sep 10, 2014 at 10:28:46AM -0400, Jeff Layton wrote:
> > Signed-off-by: Jeff Layton <jlayton@primarydata.com>
> > ---
> > fs/nfs/delegation.c | 37 +++++++++++++++++++++----------------
> > fs/nfs/nfs4state.c | 24 +++++++++++++++---------
> > fs/nfs/pagelist.c | 3 ++-
> > fs/nfs/write.c | 39 +++++++++++++++++++++++++++++++++------
> > 4 files changed, 71 insertions(+), 32 deletions(-)
> >
> > diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> > index 5853f53db732..22c6eed9bb5b 100644
> > --- a/fs/nfs/delegation.c
> > +++ b/fs/nfs/delegation.c
> > @@ -85,25 +85,30 @@ static int nfs_delegation_claim_locks(struct nfs_open_context *ctx, struct nfs4_
> > {
> > struct inode *inode = state->inode;
> > struct file_lock *fl;
> > + struct file_lock_context *flctx = inode->i_flctx;
> > + struct list_head *list;
> > int status = 0;
> >
> > - if (inode->i_flock == NULL)
> > - goto out;
> > -
> > - /* Protect inode->i_flock using the i_lock */
> > - spin_lock(&inode->i_lock);
> > - for (fl = inode->i_flock; fl != NULL; fl = fl->fl_next) {
> > - if (!(fl->fl_flags & (FL_POSIX|FL_FLOCK)))
> > - continue;
> > - if (nfs_file_open_context(fl->fl_file) != ctx)
> > - continue;
> > - spin_unlock(&inode->i_lock);
> > - status = nfs4_lock_delegation_recall(fl, state, stateid);
> > - if (status < 0)
> > - goto out;
> > - spin_lock(&inode->i_lock);
> > + flctx = inode->i_flctx;
> > + if (flctx) {
> > + list = &flctx->flc_posix;
> > + spin_lock(&flctx->flc_lock);
> > +restart:
> > + list_for_each_entry(fl, list, fl_list) {
> > + if (nfs_file_open_context(fl->fl_file) != ctx)
> > + continue;
> > + spin_unlock(&flctx->flc_lock);
> > + status = nfs4_lock_delegation_recall(fl, state, stateid);
> > + if (status < 0)
> > + goto out;
> > + spin_lock(&flctx->flc_lock);
> > + }
> > + if (list == &flctx->flc_posix) {
> > + list = &flctx->flc_flock;
> > + goto restart;
> > + }
> > + spin_unlock(&flctx->flc_lock);
> > }
> > - spin_unlock(&inode->i_lock);
> > out:
> > return status;
> > }
> > diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
> > index a043f618cd5a..2899a0f26293 100644
> > --- a/fs/nfs/nfs4state.c
> > +++ b/fs/nfs/nfs4state.c
> > @@ -1377,21 +1377,23 @@ static int nfs4_reclaim_locks(struct nfs4_state *state, const struct nfs4_state_
> > struct inode *inode = state->inode;
> > struct nfs_inode *nfsi = NFS_I(inode);
> > struct file_lock *fl;
> > + struct file_lock_context *flctx = inode->i_flctx;
> > + struct list_head *list;
> > int status = 0;
> >
> > - if (inode->i_flock == NULL)
> > + if (!flctx)
> > return 0;
> >
> > + list = &flctx->flc_posix;
> > +
> > /* Guard against delegation returns and new lock/unlock calls */
> > down_write(&nfsi->rwsem);
> > - /* Protect inode->i_flock using the BKL */
> > - spin_lock(&inode->i_lock);
> > - for (fl = inode->i_flock; fl != NULL; fl = fl->fl_next) {
> > - if (!(fl->fl_flags & (FL_POSIX|FL_FLOCK)))
> > - continue;
> > + spin_lock(&flctx->flc_lock);
> > +restart:
> > + list_for_each_entry(fl, list, fl_list) {
> > if (nfs_file_open_context(fl->fl_file)->state != state)
> > continue;
> > - spin_unlock(&inode->i_lock);
> > + spin_unlock(&flctx->flc_lock);
> > status = ops->recover_lock(state, fl);
> > switch (status) {
> > case 0:
> > @@ -1418,9 +1420,13 @@ static int nfs4_reclaim_locks(struct nfs4_state *state, const struct nfs4_state_
> > /* kill_proc(fl->fl_pid, SIGLOST, 1); */
> > status = 0;
> > }
> > - spin_lock(&inode->i_lock);
> > + spin_lock(&flctx->flc_lock);
> > }
> > - spin_unlock(&inode->i_lock);
> > + if (list == &flctx->flc_posix) {
> > + list = &flctx->flc_flock;
> > + goto restart;
> > + }
> > + spin_unlock(&flctx->flc_lock);
> > out:
> > up_write(&nfsi->rwsem);
> > return status;
> > diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
> > index ba491926df5f..4df8d8755026 100644
> > --- a/fs/nfs/pagelist.c
> > +++ b/fs/nfs/pagelist.c
> > @@ -782,7 +782,8 @@ static bool nfs_can_coalesce_requests(struct nfs_page *prev,
> > if (prev) {
> > if (!nfs_match_open_context(req->wb_context, prev->wb_context))
> > return false;
> > - if (req->wb_context->dentry->d_inode->i_flock != NULL &&
> > + if (req->wb_context->dentry->d_inode->i_flctx != NULL &&
> > + !list_empty(&req->wb_context->dentry->d_inode->i_flctx->flc_posix) &&
> > !nfs_match_lock_context(req->wb_lock_context,
> > prev->wb_lock_context))
> > return false;
> > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > index e3b5cf28bdc5..02b8777f8f2f 100644
> > --- a/fs/nfs/write.c
> > +++ b/fs/nfs/write.c
> > @@ -1128,7 +1128,8 @@ int nfs_flush_incompatible(struct file *file, struct page *page)
> > do_flush = req->wb_page != page || req->wb_context != ctx;
> > /* for now, flush if more than 1 request in page_group */
> > do_flush |= req->wb_this_page != req;
> > - if (l_ctx && ctx->dentry->d_inode->i_flock != NULL) {
> > + if (l_ctx && ctx->dentry->d_inode->i_flctx &&
> > + !list_empty(&ctx->dentry->d_inode->i_flctx->flc_posix)) {
> > do_flush |= l_ctx->lockowner.l_owner != current->files
> > || l_ctx->lockowner.l_pid != current->tgid;
> > }
> > @@ -1189,6 +1190,12 @@ out:
> > return PageUptodate(page) != 0;
> > }
> >
> > +static bool
> > +is_whole_file_wrlock(struct file_lock *fl)
> > +{
> > + return fl->fl_start == 0 && fl->fl_end == OFFSET_MAX && fl->fl_type == F_WRLCK;
> > +}
> > +
> > /* If we know the page is up to date, and we're not using byte range locks (or
> > * if we have the whole file locked for writing), it may be more efficient to
> > * extend the write to cover the entire page in order to avoid fragmentation
> > @@ -1199,17 +1206,37 @@ out:
> > */
> > static int nfs_can_extend_write(struct file *file, struct page *page, struct inode *inode)
> > {
> > + int ret;
> > + struct file_lock_context *flctx = inode->i_flctx;
> > + struct file_lock *fl;
> > +
> > if (file->f_flags & O_DSYNC)
> > return 0;
> > if (!nfs_write_pageuptodate(page, inode))
> > return 0;
> > if (NFS_PROTO(inode)->have_delegation(inode, FMODE_WRITE))
> > return 1;
> > - if (inode->i_flock == NULL || (inode->i_flock->fl_start == 0 &&
> > - inode->i_flock->fl_end == OFFSET_MAX &&
> > - inode->i_flock->fl_type != F_RDLCK))
>
> Doesn't the existing code already have a bug? Without the i_lock
> inode->i_flock could turn NULL partyway through
>
> There's a bug in the existing code, isn't there? Without holding the
> i_lock, couldn't inode->i_flock turn NULL partway through this
> conditional and cause NULL dereferences? (Or, more bizarrely, the
> checks of those various fields could end up being for different locks.)
>

(cc'ing Trond and Scott...)

Yeah, I think you're correct. We really ought to hold the i_lock there
once we see that i_flock isn't NULL.

It's stuff like this that makes me wonder if we ought to convert all of
this to using RCU. Being able to hold the rcu_read_lock instead of the
i_lock (or the flc_lock once the conversion is done) would be rather
nice.

> > - return 1;
> > - return 0;
> > + /* no lock context == no locks */
> > + if (!flctx)
> > + return 0;
> > +
> > + /* if lists are empty then there are no locks */
> > + if (list_empty(&flctx->flc_posix) && list_empty(&flctx->flc_flock))
> > + return 0;
> > +
> > + ret = 0;
> > + /* Check to see if there are whole file write locks */
> > + spin_lock(&flctx->flc_lock);
> > + fl = list_first_entry(&flctx->flc_posix, struct file_lock, fl_list);
> > + if (is_whole_file_wrlock(fl)) {
> > + ret = 1;
> > + } else {
> > + fl = list_first_entry(&flctx->flc_flock, struct file_lock, fl_list);
> > + if (is_whole_file_wrlock(fl))
> > + ret = 1;
> > + }
> > + spin_unlock(&flctx->flc_lock);
> > + return ret;
>
> Kind of pity we're turning 5 lines of code into 20 in the name of
> simplification. Could be slightly pithier:
>
> ret = is_whole_file_wrlock(fl);
> if (!ret) {
> fl = ...
> ret = is_whole_file_wrlock(fl);
> }
>
> But, whatever, looks OK to me.
>
> --b.
>

Yes, that's the downside of moving to multiple list_heads. Still, I
think it's worth doing that even if we end up with the code a bit more
verbose.

It may be best to consider moving some of this into helpers that live
in locks.c. I really don't like having filesystems poke around in the
intimate details of the file locking code as a general rule...

> > }
> >
> > /*
> > --
> > 1.9.3
> >

--
Jeff Layton <jlayton@primarydata.com>