Re: [RFC v5 1/2] epoll: Implement eventpoll_replace_file()

[Christian Brauner]

On Sat, Apr 29, 2023 at 05:49:54AM +0000, aloktiagi wrote:
> Introduce a mechanism to replace a file linked in the epoll interface with a new
> file.
> 
> eventpoll_replace() finds all instances of the file to be replaced and replaces
> them with the new file and the interested events.
> 
> Signed-off-by: aloktiagi <aloktiagi@xxxxxxxxx>
> ---
> Changes in v5:
>   - address review comments and move the call to replace old file in each
>     subsystem (epoll, io_uring, etc.) outside the fdtable helpers like
>     replace_fd().
> 
> Changes in v4:
>   - address review comment to remove the redundant eventpoll_replace() function.
>   - removed an extra empty line introduced in include/linux/file.h
> 
> Changes in v3:
>   - address review comment and iterate over the file table while holding the
>     spin_lock(&files->file_lock).
>   - address review comment and call filp_close() outside the
>     spin_lock(&files->file_lock).
> ---
>  fs/eventpoll.c            | 65 +++++++++++++++++++++++++++++++++++++++
>  include/linux/eventpoll.h |  8 +++++
>  2 files changed, 73 insertions(+)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 64659b110973..be9d192b223d 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -935,6 +935,71 @@ void eventpoll_release_file(struct file *file)
>  	mutex_unlock(&epmutex);
>  }
>  
> +static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
> +			struct file *tfile, int fd, int full_check);
> +
> +/*
> + * This is called from eventpoll_replace() to replace a linked file in the epoll
> + * interface with a new file received from another process. This is useful in
> + * cases where a process is trying to install a new file for an existing one
> + * that is linked in the epoll interface
> + */
> +int eventpoll_replace_file(struct file *toreplace, struct file *file, int tfd)
> +{
> +	int fd;
> +	int error = 0;
> +	struct eventpoll *ep;
> +	struct epitem *epi;
> +	struct hlist_node *next;
> +	struct epoll_event event;
> +	struct hlist_head *to_remove = toreplace->f_ep;
> +
> +	if (!file_can_poll(file))
> +		return 0;
> +
> +	mutex_lock(&epmutex);

Sorry, I missed that you send a new version somehow.

So, I think I mentioned this last time: The locking has changed to
reduce contention on the global mutex. Both epmutex and ep_remove() are
gone. So this doesn't even compile anymore...

  CC      fs/eventpoll.o
../fs/eventpoll.c: In function ‘eventpoll_replace_file’:
../fs/eventpoll.c:998:21: error: ‘epmutex’ undeclared (first use in this function); did you mean ‘mutex’?
  998 |         mutex_lock(&epmutex);
      |                     ^~~~~~~
      |                     mutex
../fs/eventpoll.c:998:21: note: each undeclared identifier is reported only once for each function it appears in
../fs/eventpoll.c:1034:17: error: implicit declaration of function ‘ep_remove’; did you mean ‘idr_remove’? [-Werror=implicit-function-declaration]
 1034 |                 ep_remove(ep, epi);
      |                 ^~~~~~~~~
      |                 idr_remove

on current mainline. So please send a new version for this.