Re: [PATCH v2] inotify: Extend ioctl to allow to request id of new watch descriptor

2018-02-09 23:45 GMT+01:00 Kirill Tkhai [off-list ref]:

On 09.02.2018 23:56, Andrew Morton wrote:

quoted

On Fri, 9 Feb 2018 18:04:54 +0300 Kirill Tkhai [off-list ref] wrote:

quoted

Watch descriptor is id of the watch created by inotify_add_watch().
It is allocated in inotify_add_to_idr(), and takes the numbers
starting from 1. Every new inotify watch obtains next available
number (usually, old + 1), as served by idr_alloc_cyclic().

CRIU (Checkpoint/Restore In Userspace) project supports inotify
files, and restores watched descriptors with the same numbers,
they had before dump. Since there was no kernel support, we
had to use cycle to add a watch with specific descriptor id:

     while (1) {
             int wd;

             wd = inotify_add_watch(inotify_fd, path, mask);
             if (wd < 0) {
                     break;
             } else if (wd == desired_wd_id) {
                     ret = 0;
                     break;
             }

             inotify_rm_watch(inotify_fd, wd);
     }

(You may find the actual code at the below link:
 https://github.com/checkpoint-restore/criu/blob/v3.7/criu/fsnotify.c#L577)

Well using a ioctl command to force a specific wd is possible, but
isn't it also possible
to do a "freeze" of all (inotify) watches which are involved, and
"unfreeze" when restoring?

Stef