Re: [PATCH v2] nvme-multipath: Early exit if no path is available

[Hannes Reinecke]

On 2/1/21 10:40 AM, Chao Leng wrote:
> On 2021/2/1 16:57, Hannes Reinecke wrote:
> > On 2/1/21 9:47 AM, Chao Leng wrote:
> > > On 2021/2/1 15:29, Hannes Reinecke wrote:[ .. ]
> > > > Urgh. Please, no. That is well impossible to debug.
> > > > Can you please open-code it to demonstrate where the difference to 
> > > > the current (and my fixed) versions is?
> > > > I'm still not clear where the problem is once we applied both patches.
> > > For example assume the list has three path, and all path is not 
> > > NVME_ANA_OPTIMIZED:
> > > head->next = ns1;
> > > ns1->next = ns2;
> > > ns2->next = head;
> > > old->next = ns2;
> > And this is where I have issues with.
> > Where does 'old' come from?
> > Clearly it was part of the list at one point; so what happened to it?
> I explained this earlier.
> In nvme_ns_remove, there is a hole between list_del_rcu and
> nvme_mpath_clear_current_path. If head->current_path is the "old", and
> the "old" is removing. The "old" is already removed from the list by
> list_del_rcu, but head->current_path is not clear to NULL by
> nvme_mpath_clear_current_path.
> Find path is race with nvme_ns_remove, use the "old" pass to
> nvme_round_robin_path to find path.

Ah. So this should be better:

@@ -202,10 +202,12 @@ static struct nvme_ns *__nvme_find_path(struct 
nvme_ns_head *head, int node)
 static struct nvme_ns *nvme_next_ns(struct nvme_ns_head *head,
                struct nvme_ns *ns)
 {
-       ns = list_next_or_null_rcu(&head->list, &ns->siblings, struct 
nvme_ns,
-                       siblings);
-       if (ns)
-               return ns;
+       if (ns && !test_bit(NVME_NS_REMOVING, &ns->flags)) {
+               ns = list_next_or_null_rcu(&head->list, &ns->siblings,
+                                          struct nvme_ns, siblings);
+               if (ns)
+                       return ns;
+       }
        return list_first_or_null_rcu(&head->list, struct nvme_ns, 
siblings);
 }

The 'NVME_NS_REMOVING' bit is set before list_del_rcu() is called, so it 
should guard against the issue you mentioned.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
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