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SubjectRe: fsnotify_mark_srcu wtf?
On Wed, Nov 9, 2016 at 1:10 PM, Jan Kara <> wrote:
> On Sun 06-11-16 08:45:54, Amir Goldstein wrote:
>> On Sat, Nov 5, 2016 at 11:34 PM, Jan Kara <> wrote:
>> > On Wed 02-11-16 23:09:26, Miklos Szeredi wrote:
>> >> We've got a report where a fanotify daemon that implements permission checks
>> >> screws up and doesn't send a reply. This then causes widespread hangs due to
>> >> fsnotify_mark_srcu read side lock being held and thus causing synchronize_srcu()
>> >> called from e.g. inotify_release()-> fsnotify_destroy_group()->
>> >> fsnotify_mark_destroy_list() to block.
>> >
>> > Yes. But if a program implementing permission checks does not reply, your
>> > system is likely hosed anyway. We can only try to somewhat limit the
>> > damage...
>> >
>> That was my initial thought as well, but at least with the sample code
>> Miklos sent
>> the only thing that gets hosed is the one process watching that one file.
>> You could think of a use case of fanotify being used to watch over files
>> in a specific user directory, where the damage on the entire system
>> should/could be limited. No?
> Yes, the damage could at least theoretically be limited only to those files
> / dirs watched by the buggy process.
>> >> Below program demonstrates the issue. It should output a single line:
>> >>
>> >> close(inotify_fd): success
>> >>
>> >> Instead it outputs nothing, which means that close(inotify_fd) got blocked by
>> >> the waiting permission event.
>> >>
>> >> Wouldn't making the srcu per-group fix this? Would that be too expensive?
>> >
>> > Per-group would be IMHO too expensive. You can have lots of groups and I'm
>> > not sure srcu would scale to that. Furthermore the SRCU protects the list
>> > of groups that need to get notification so it would not even be easily
>> > possible. Also Amir's solution is buggy - I'll comment on that as a reply
>> > to his patch. I'll try to find something to improve the situation but so
>> > far I have no good idea...
>> >
>> Yes, very much buggy indeed :/
>> Anyway, the reason I drafted it quickly was to highlight the fact that the
>> marks only need to live to the point of decision whether or not the event
>> should be sent to the group and afterwards, its sufficient to grab the
>> group reference, without having impact on the entire system.
> Yes, fanotify code as such does not need the marks anymore. But the core
> fsnotify code does...
>> Yet another possible ugly (but less buggy) solution would be
>> to iterate all marks under SRCU read protection.
>> If any group is about to block (either by suggested return value
>> EAGAIN or another
>> by using a new op should_handle_event_deferred), defer event handling to post
>> marks iteration, by keeping a few group references on stack.
> And this does not work as well... Fanotify must notify groups by their
> priority so you cannot arbitrarily reorder ordering in which groups get
> notified. I'm currently pondering on using mark refcount to pin it when
> processing permission event but there are still some details to check.

All right, mark refcount sound like the proper solution.

In case this approach doesn't work out for some reason, you may want
to consider fsnotify_mark_srcu (and destroy_list) per priority.
Or just 2 srcu, one for for priority 0 and one for other.
Because priority > 0 may block and priority 0 may not block.

The priority set on an inode/vfsmount can be easily encoded into the
i_fsnotify_mask/mnt_fsnotify_mask, e.g.:
#define FS_GROUP_PRIO_1 0x00040000 /* fanotify content
based access control */
#define FS_GROUP_PRIO_2 0x00080000 /* fanotify
pre-content access */

in fsnotify(), only need to take the read side srcu of relevant priority groups,
but need to take extra care to set the priority bit in the inode/mnt
mask *before*
adding the mark to the list, with a relevant memory barrier before checking
the priority bits in fsnotify().


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