Hmm, it looks like this IS still an issue upstream. Do you know why an upstream patch didn’t get submitted?

I think it could also be fixed by moving the mdcache_lru_insert above cih_set_latched (since that no longer can fail).

Frank

From: Malahal Naineni [mailto:malahal@gmail.com]
Sent: Thursday, April 16, 2020 9:08 PM
To: Pradeep Thomas <pradeepthomas@gmail.com>
Cc: nfs-ganesha <devel@lists.nfs-ganesha.org>
Subject: [NFS-Ganesha-Devel] Re: Hitting a crash in mdcache_lru_cleanup_push.

Someone (from amazon?) reported this issue but didn't follow up with a patch. So we ended up with a patch in our code base. See commit ce4c049b0871eaae78325bb30a1ef1df3a14e77c in our repo (https://github.com/ganltc/nfs-ganesha.git branch ibm2.7).

Regards, Malahal.

On Fri, Apr 17, 2020 at 6:29 AM Pradeep Thomas <pradeepthomas@gmail.com> wrote:

Hello Daniel/Frank,

While debugging a crash from 2.7.1 Ganesha, I see a potential race between the two paths below:

Thread 1 (waiting for the qlock to insert to LRU)
nfs4_mds_putfh -> mdcache_create_handle -> mdcache_locate_host -> mdcache_new_entry -> mdcache_lru_insert -> lru_insert_entry

Thread 2 (unlink the same object) - since the object is already in mdcache at this point, I believe other threads will get it.
fsal_remove -> mdcache_unlink -> _mdc_unreachable -> _mdcache_kill_entry -> mdcache_lru_cleanup_push

The second thread will find the lru something like this:
$5 = {q = {next = 0x0, prev = 0x0}, qid = LRU_ENTRY_NONE, refcnt = 2, flags = 0, lane = 12, cf = 0}

So, the below code will end up crashing:
        if (!(lru->qid == LRU_ENTRY_CLEANUP)) {
                struct lru_q *q;

                /* out with the old queue */
                q = lru_queue_of(entry); <<-- q will be NULL because qid == LRU_ENTRY_NONE

Should Thread 2 just ignore if q is NULL and let Thread1's operation to free the entry later?

Also, please let me know if there is any recent fixes in this area.

Thanks,
Pradeep
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