Re: [PATCH] fuse: fix race between inode/dentry invalidation and readdir

[Luis Henriques <luis@igalia.com>]

On Mon, Apr 27 2026, Joanne Koong wrote:

> On Mon, Apr 27, 2026 at 10:23 AM Luis Henriques <luis@igalia.com> wrote:
>>
>> Hi Joanne!
>>
>> On Fri, Apr 24 2026, Joanne Koong wrote:
>>
>> > On Fri, Apr 24, 2026 at 6:53 AM Luis Henriques <luis@igalia.com> wrote:
>> >>
>> >> When there's a readdir in progress, doing a FUSE_NOTIFY_INVAL_{INODE,ENTRY}
>> >> on an inode or dentry may result in stale directory info being cached.  This
>> >> is because the invalidation does not reset the readdir cache.
>> >>
>> >> This patch fixes this issue by adding a call to fuse_rdc_reset() (modified
>> >> to include the required locking) to these two operations, allowing the
>> >> readdir cache to be invalidated while it's being filled-in.
>> >
>> > Hi Luis,
>> >
>> > Just curious, are you hitting this issue in practice or is this mostly
>> > theoretical?
>> >
>> > afaict for fuse_notify_inval_entry(), it calls into
>> > fuse_reverse_inval_entry() -> fuse_dir_changed(parent), which calls
>> > inode_maybe_inc_iversion(). afaict, this actually increments i_version
>> > (since I_VERSION_QUERIED flag was set when the cache's iversion was
>> > initialized with inode_query_iversion() in fuse_readdir_cached()),
>> > which means the next readdir call will detect this version change and
>> > call fuse_rdc_reset() (in fuse_readdir_cached()). I'm not sure I see
>> > how this leads to stale directory info lingering in the cache after a
>> > concurrent fuse_notify_inval_entry()?
>> >
>> > For teh fuse_notify_inval_inode() case, which I'm assuming is the case
>> > you're running into where the directory is the inode being
>> > invalidated, I see the call to fuse_reverse_inval_inode() which calls
>> > invalidate_inode_pages2_range() if the offset was non-negative, which
>> > will invalidate the readdir cache's pages, which means on the next
>> > readdir call, will already call fuse_rdc_reset() when it detects the
>> > missing page in the cache (in fuse_readdir_cached()). So I'm not
>> > really seeing how this can happen either for the
>> > fuse_notify_inval_inode() case? Unless you are passing a negative
>> > offset, but as I understand it, passing a negative offset is used only
>> > if the server wants attributes invalidated [1], not any data.
>> >
>> > afaics, the onlyy stale directory info returned would be for the case
>> > for a concurrent readdir that has already passed the pos == 0
>> > iversion/mtime check when the invalidation arrives, but that seems
>> > like a server synchronization issue, eg if the server wants uptodate
>> > data when doing a concurrent readdir and invalidation, they have to
>> > order that themselves. ANy fresh lookup after that though, I think
>> > wouldalways return fresh/non-stale data for the reasons mentioned
>> > above.
>> >
>> > Does this align with what you're seeing in the code or am I missing
>> > something here?
>
> Hi Luis,
>
>>
>> First of all, thanks a lot for looking into this and for doing such a
>> great description of the issue.
>>
>> So, I did had a report regarding a possible race between a readdir and
>> invalidation when using keep_cache and cache_readdir.  But, unfortunately,
>> I don't have a lot of information regarding the actual issue, and it isn't
>> something reproducible.
>>
>> Then, looking at the code (and, for full-disclosure, I've also looked at a
>> claude analysis that was handed over to me) I could see a race that I'm
>> trying to fix with this patch.  But I believe it's the race that you claim
>> above that it's a server synchronisation problem.  For example, with a
>> NOTIFY_INVAL_INODE operation, when fuse_reverse_inval_inode() is called
>> while fuse_add_dirent_to_cache() is being executed in parallel, the
>> iversion/mtime update could be missed.
>>
>> It is possible to hit this small race by instrumenting the code, and I
>> could occasionally (and momentarily) see stale data while running readdir
>> in such instrumented testing environment.  Do you think that's something
>> inherent to the usage of the INVAL_INODE op, and this race will need to be
>> handled by user-space?
>
> imo yes, that is not a bug in the kernel and userspace is responsible
> for synchronizing/coordinating that. I think the kernel is just
> responsible for ensuring that any subsequent readdirs are not stale,
> but afaict the existing code handles that.

Ack, thanks.

>> In fact, the report I got seemed to indicate that the issue was not going
>> away with a fresh lookup (though an 'echo 1 > /proc/sys/vm/drop_cache'
>> would fix it).  But maybe that's another indication that this is a problem
>> in the user-space server.
>
> that seems weird to me, maybe there's something else at play here in
> addition to the concurrent race? Is there a repro for where the stale
> data survives a fresh lookup?

Unfortunately no, I do not have any reproducer.  And from looking at the
code I couldn't find anything else.  I'll have to look closer into the
user-space code doing the invalidation to try to understand what else
could be at play here.  And again, thank you for your feedback, Joanne.

Cheers,
-- 
Luís