4:40 p.m.
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(a)gmail.com>
Cc: nfs-ganesha <devel(a)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(a)gmail.com
<mailto: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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