Re: [PATCH 3/6] fuse: wait for end of IO on release

[Miklos Szeredi]

On Thu, Aug 21, 2014 at 6:08 PM, Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx> wrote:
> There are two types of I/O activity that can be "in progress" at the time
> of fuse_release() execution: asynchronous read-ahead and write-back. The
> patch ensures that they are completed before fuse_release_common sends
> FUSE_RELEASE to userspace.
>
> So far as fuse_release() waits for end of async I/O, its callbacks
> (fuse_readpages_end and fuse_writepage_finish) calling fuse_file_put cannot
> be the last holders of fuse file anymore. To emphasize the fact, the patch
> replaces fuse_file_put with __fuse_file_put there.

1) spinlock around __fuse_file_put() is unnecessary,
wake_up/wait_event will provide the necessary synchronization.
2) can't we always wait for I/O and just make the actual RELEASE
message sync or async based on the flag?
3) and can't we merge the fuseblk case into this as well?

Thanks,
Miklos

>
> Signed-off-by: Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx>
> ---
>  fs/fuse/file.c |   76 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 71 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 7723b3f..73bce1b 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -149,6 +149,17 @@ static void fuse_file_put(struct fuse_file *ff, bool sync)
>         }
>  }
>
> +/*
> + * Asynchronous callbacks may use it instead of fuse_file_put() because
> + * we guarantee that they are never last holders of ff. Hitting BUG() below
> + * will make clear any violation of the guarantee.
> + */
> +static void __fuse_file_put(struct fuse_file *ff)
> +{
> +       if (atomic_dec_and_test(&ff->count))
> +               BUG();
> +}
> +
>  int fuse_do_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
>                  bool isdir)
>  {
> @@ -279,6 +290,11 @@ static void fuse_prepare_release(struct fuse_file *ff, int flags, int opcode)
>         req->in.args[0].value = inarg;
>  }
>
> +static bool must_release_synchronously(struct fuse_file *ff)
> +{
> +       return ff->open_flags & FOPEN_SYNC_RELEASE;
> +}
> +
>  void fuse_release_common(struct file *file, int opcode)
>  {
>         struct fuse_file *ff;
> @@ -302,6 +318,13 @@ void fuse_release_common(struct file *file, int opcode)
>         req->misc.release.path = file->f_path;
>
>         /*
> +        * No more in-flight asynchronous READ or WRITE requests if
> +        * fuse file release is synchronous
> +        */
> +       if (must_release_synchronously(ff))
> +               BUG_ON(atomic_read(&ff->count) != 1);
> +
> +       /*
>          * Normally this will send the RELEASE request, however if
>          * some asynchronous READ or WRITE requests are outstanding,
>          * the sending will be delayed.
> @@ -321,11 +344,34 @@ static int fuse_open(struct inode *inode, struct file *file)
>  static int fuse_release(struct inode *inode, struct file *file)
>  {
>         struct fuse_conn *fc = get_fuse_conn(inode);
> +       struct fuse_file *ff = file->private_data;
>
>         /* see fuse_vma_close() for !writeback_cache case */
>         if (fc->writeback_cache)
>                 write_inode_now(inode, 1);
>
> +       if (must_release_synchronously(ff)) {
> +               struct fuse_inode *fi = get_fuse_inode(inode);
> +
> +               /*
> +                * Must remove file from write list. Otherwise it is possible
> +                * this file will get more writeback from another files
> +                * rerouted via write_files.
> +                */
> +               spin_lock(&ff->fc->lock);
> +               list_del_init(&ff->write_entry);
> +               spin_unlock(&ff->fc->lock);
> +
> +               wait_event(fi->page_waitq, atomic_read(&ff->count) == 1);
> +
> +               /*
> +                * spin_unlock_wait(&ff->fc->lock) would be natural here to
> +                * wait for threads just released ff to leave their critical
> +                * sections. But taking spinlock is the first thing
> +                * fuse_release_common does, so that this is unnecessary.
> +                */
> +       }
> +
>         fuse_release_common(file, FUSE_RELEASE);
>
>         /* return value is ignored by VFS */
> @@ -823,8 +869,17 @@ static void fuse_readpages_end(struct fuse_conn *fc, struct fuse_req *req)
>                 unlock_page(page);
>                 page_cache_release(page);
>         }
> -       if (req->ff)
> -               fuse_file_put(req->ff, false);
> +       if (req->ff) {
> +               if (must_release_synchronously(req->ff)) {
> +                       struct fuse_inode *fi = get_fuse_inode(req->inode);
> +
> +                       spin_lock(&fc->lock);
> +                       __fuse_file_put(req->ff);
> +                       wake_up(&fi->page_waitq);
> +                       spin_unlock(&fc->lock);
> +               } else
> +                       fuse_file_put(req->ff, false);
> +       }
>  }
>
>  struct fuse_fill_data {
> @@ -851,6 +906,7 @@ static void fuse_send_readpages(struct fuse_fill_data *data)
>         if (fc->async_read) {
>                 req->ff = fuse_file_get(ff);
>                 req->end = fuse_readpages_end;
> +               req->inode = data->inode;
>                 fuse_request_send_background(fc, req);
>         } else {
>                 fuse_request_send(fc, req);
> @@ -1502,7 +1558,7 @@ static void fuse_writepage_free(struct fuse_conn *fc, struct fuse_req *req)
>         for (i = 0; i < req->num_pages; i++)
>                 __free_page(req->pages[i]);
>
> -       if (req->ff)
> +       if (req->ff && !must_release_synchronously(req->ff))
>                 fuse_file_put(req->ff, false);
>  }
>
> @@ -1519,6 +1575,8 @@ static void fuse_writepage_finish(struct fuse_conn *fc, struct fuse_req *req)
>                 dec_zone_page_state(req->pages[i], NR_WRITEBACK_TEMP);
>                 bdi_writeout_inc(bdi);
>         }
> +       if (must_release_synchronously(req->ff))
> +               __fuse_file_put(req->ff);
>         wake_up(&fi->page_waitq);
>  }
>
> @@ -1659,8 +1717,16 @@ int fuse_write_inode(struct inode *inode, struct writeback_control *wbc)
>
>         ff = __fuse_write_file_get(fc, fi);
>         err = fuse_flush_times(inode, ff);
> -       if (ff)
> -               fuse_file_put(ff, 0);
> +       if (ff) {
> +               if (must_release_synchronously(ff)) {
> +                       spin_lock(&fc->lock);
> +                       __fuse_file_put(ff);
> +                       wake_up(&fi->page_waitq);
> +                       spin_unlock(&fc->lock);
> +
> +               } else
> +                       fuse_file_put(ff, false);
> +       }
>
>         return err;
>  }
>
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