Re: [PATCHv2 05/17] nvme: add Clang context annotations for nvme_ns_head::current_path

[Nilay Shroff <nilay@linux.ibm.com>]

On 6/29/26 11:30 AM, Marco Elver wrote:
> On Sun, 28 Jun 2026 at 18:07, Paul E. McKenney <paulmck@kernel.org> wrote:
>>
>> On Sun, Jun 28, 2026 at 08:00:00AM +0200, Marco Elver wrote:
>>> On Sat, 27 Jun 2026 at 19:14, Paul E. McKenney <paulmck@kernel.org> wrote:
>>>>
>>>> On Sat, Jun 27, 2026 at 05:38:44PM +0200, Marco Elver wrote:
>>>>> On Fri, 26 Jun 2026 at 20:36, Paul E. McKenney <paulmck@kernel.org> wrote:
>>>>>> On Fri, Jun 26, 2026 at 08:40:50AM +0200, Christoph Hellwig wrote:
>>>>>>> On Sun, Jun 14, 2026 at 06:45:20PM +0530, Nilay Shroff wrote:
>>>>>>>> +++ b/drivers/nvme/host/multipath.c
>>>>>>>> @@ -231,8 +231,16 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
>>>>>>>>      bool changed = false;
>>>>>>>>      int node;
>>>>>>>>
>>>>>>>> +   /*
>>>>>>>> +    * This helper is used by namespace failover/teardown and I/O policy
>>>>>>>> +    * update paths. We only compare the head->current_path[] pointer value
>>>>>>>> +    * and do not dereference the referenced namespace, so suppress the
>>>>>>>> +    * context analysis warning for this lockless inspection of the
>>>>>>>> +    * __rcu_guarded pointer.
>>>>>>>> +    */
>>>>>>>>      for_each_node(node) {
>>>>>>>> -           if (ns == rcu_access_pointer(head->current_path[node])) {
>>>>>>>> +           if (context_unsafe(ns ==
>>>>>>>> +                           rcu_access_pointer(head->current_path[node]))) {
>>>>>>>
>>>>>>> I think we need a helper for this, as for a simple pointer value
>>>>>>> comparison without a dereference we don't really need either
>>>>>>> rcu_access_pointer nor locking.
>>>>>>>
>>>>>>> Maybe somthing like a
>>>>>>>
>>>>>>>        rcu_compare_pointer(rcu_pointer, nonrcu_pointer)
>>>>>>>
>>>>>>> ?
>>>>>>
>>>>>> We could provide something like this:
>>>>>>
>>>>>> #define rcu_compare_pointer(rcu_pointer, nonrcu_pointer) \
>>>>>>          context_unsafe(rcu_access_pointer(rcu_pointer) == (nonrcu_pointer))
>>>>>>
>>>>>> Or maybe rcu_pointer_equals()?
>>>>>>
>>>>>> Marco, thoughts?
>>>>>
>>>>> I see 2 options:
>>>>>
>>>>> 1. rcu_compare_pointer / rcu_pointer_equals as proposed. It's more
>>>>> explicit but does add some friction during Context Analysis
>>>>> enablement. But this only makes sense if there are cases where
>>>>> rcu_access_pointer() should be used under the RCU reader lock, which
>>>>> led me to the next suggestion...
>>>>>
>>>>> 2. Redefine rcu_access_pointer() to just not require the RCU read-side
>>>>> lock to be held as:
>>>>>
>>>>>> #define rcu_access_pointer(p) context_unsafe(__rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu))
>>>>>
>>>>> [ Or alternatively wrap __rcu_access_pointer() in context_unsafe()
>>>>>    (similar to rcu_assign_pointer and friends). ]
>>>>>
>>>>> I think rcu_access_pointer() is in the same category as the other RCU
>>>>> pointer helpers, although currently only the pointer update helpers
>>>>> imply context_unsafe(); I think it might have been an oversight
>>>>> initially, and we should change rcu_access_pointer() to match. There
>>>>> should be no reason why an rcu_access_pointer() should be protected by
>>>>> the RCU read-side critical section, since it's not meant for
>>>>> dereferencing the pointer (that'd be a bug; its documentation also
>>>>> says "Return the value of the specified RCU-protected pointer, but
>>>>> omit the lockdep checks for being in an RCU read-side critical section
>>>>> [...]").
>>>>>
>>>>> If you agree there should be no cases where an rcu_access_pointer()
>>>>> should be guarded by an RCU read-side critical section, then I think
>>>>> this is the simplest and correct design, and avoids expanding the RCU
>>>>> API.
>>>>
>>>> I don't know of any uses of rcu_access_pointer() within an RCU read-side
>>>> critical section, but code in a function that might be called both within
>>>> and outside of a critical section might well use rcu_access_pointer().
>>>> In other words, it should be OK to use rcu_access_pointer() within an
>>>> RCU read-side critical section as well as outside of one.
>>>
>>> Thanks, yes, that's what I wanted to confirm; i.e. it's ok to use
>>> rcu_access_pointer() within and outside an RCU read-side critical
>>> section.
>>>
>>> In which case, my proposal (2) to make rcu_access_pointer() not warn
>>> on accessing __rcu_guarded pointers outside an RCU read-side critical
>>> section should be the simpler and more generic change (vs. adding a
>>> new rcu_pointer_equals() helper).
>>
>> As shown below?
>>
>> My guess is that this change makes it unnecessary to have a separate
>> RCU-protected-pointer comparison macro, but please let me know if I am
>> missing something.
> 
> Looks good to me. Yes, this makes a new comparison macro unnecessary.
> 
> Nilay, does this work and simplify the patch for you?

Yeah, this work for me. In fact, I have already tested this patch from
Paul and sent my Tested-by tag[1]. You may have missed it.

[1] https://lore.kernel.org/all/2a3c6a56-a9a6-461e-80ca-c0f2b4203bff@linux.ibm.com/

Thanks,
--Nilay