Messages in this thread Patch in this message |  | | From | Luis Henriques <> | | Subject | Re: [PATCH] fuse: fix race between inode/dentry invalidation and readdir | | Date | Mon, 27 Apr 2026 10:23:50 +0100 |
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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?
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?
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.
Cheers, -- Luís
> >> >> Assisted-by: Claude:claude-opus-4-5 >> Signed-off-by: Luis Henriques <luis@igalia.com> >> --- >> fs/fuse/dir.c | 5 +++-- >> fs/fuse/fuse_i.h | 13 +++++++++++++ >> fs/fuse/inode.c | 1 + >> fs/fuse/readdir.c | 6 +++--- >> 4 files changed, 20 insertions(+), 5 deletions(-) >> >> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c >> index 7ac6b232ef12..6e5851de3613 100644 >> --- a/fs/fuse/dir.c >> +++ b/fs/fuse/dir.c >> @@ -1615,6 +1615,7 @@ int fuse_reverse_inval_entry(struct fuse_conn *fc, u64 parent_nodeid, >> if (!(flags & FUSE_EXPIRE_ONLY)) >> d_invalidate(entry); >> fuse_invalidate_entry_cache(entry); >> + fuse_rdc_reset(entry->d_inode); > > Hmm... I think this resets the child's readdir cache but it's the > parent's readdir cache that would have to be invalidated, so would > this have to be fuse_rdc_reset(parent)? > > Thanks, > Joanne > > [1] https://libfuse.github.io/doxygen/fuse__lowlevel_8h.html#a9cb974af9745294ff446d11cba2422f1 > >> >> if (child_nodeid != 0) { >> inode_lock(d_inode(entry)); >> @@ -1637,7 +1638,7 @@ int fuse_reverse_inval_entry(struct fuse_conn *fc, u64 parent_nodeid, >> dont_mount(entry); >> clear_nlink(d_inode(entry)); >> err = 0; >> - badentry: >> +badentry: >> inode_unlock(d_inode(entry)); >> if (!err) >> d_delete(entry);
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