Re: [PATCH 0/6 v2] nfs_localio: fixes for races and errors from older compilers

Chuck Lever
2025-05-16T15:33:30Z

On 5/10/25 3:57 PM, Mike Snitzer wrote:
> On Sat, May 10, 2025 at 12:02:27PM -0400, Chuck Lever wrote:
>> On 5/9/25 11:01 PM, NeilBrown wrote:
>>> On Sat, 10 May 2025, Chuck Lever wrote:
>>>> [ adding Paul McK ]
>>>>
>>>> On 5/8/25 8:46 PM, NeilBrown wrote:
>>>>> This is a revised version a the earlier series. I've actually tested
>>>>> this time and fixed a few issues including the one that Mike found.
>>>>
>>>> As Mike mentioned in a previous thread, at this point, any fix for this
>>>> issue will need to be applied to recent stable kernels as well. This
>>>> series looks a bit too complicated for that.
>>>>
>>>> I expect that other subsystems will encounter this issue eventually,
>>>> so it would be beneficial to address the root cause. For that purpose, I
>>>> think I like Vincent's proposal the best:
>>>>
>>>> https://lore.kernel.org/linux-nfs/8c67a295-8caa-4e53-a764-f691657bbe62@wanadoo.fr/raw
>>>>
>>>> None of this is to say that Neil's patches shouldn't be applied. But
>>>> perhaps these are not a broad solution to the RCU compilation issue.
>>>
>>> Do we need a "broad solution to the RCU compilation issue"?
>>
>> Fair question. If the current localio code is simply incorrect as it
>> stands, then I suppose the answer is no. Because gcc is happy to compile
>> it in most cases, I thought the problem was with older versions of gcc,
>> not with localio (even though, I agree, the use of an incomplete
>> structure definition is somewhat brittle when used with RCU).
>>
>>
>>> Does it ever make sense to "dereference" a pointer to a structure that is
>>> not fully specified? What does that even mean?
>>>
>>> I find it harder to argue against use of rcu_access_pointer() in that
>>> context, at least for test-against-NULL, but sparse doesn't complain
>>> about a bare test of an __rcu pointer against NULL, so maybe there is no
>>> need for rcu_access_pointer() for simple tests - in which case the
>>> documentation should be updated.
>>
>> For backporting purposes, inventing our own local RCU helper to handle
>> the situation might be best. Then going forward, apply your patches to
>> rectify the use of the incomplete structure definition, and the local
>> helper can eventually be removed.
>>
>> My interest is getting to a narrow set of changes that can be applied
>> now and backported as needed. The broader clean-ups can then be applied
>> to future kernels (or as subsequent patches in the same merge window).
>>
>> My 2 cents, worth every penny.
>
> I really would prefer we just use this patch as the stop-gap for 6.14
> and 6.15 (which I have been carrying for nearly a year now because I
> need to support an EL8 platform that uses gcc 8.5):
>
> https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/commit/?h=kernel-6.12.24/nfs-testing&id=f9add5e4c9b4629b102876dce9484e1da3e35d1f
>
> Then we work through getting Neil's patchset to land for 6.16 and
> revert the stop-gap (dummy nfsd_file) patch.
>
>>> (of course rcu_dereference() doesn't actually dereference the pointer,
>>> despite its name. It just declared that there is an imminent intention
>>> to dereference the pointer.....)
>>>
>>> NeilBrown
>
> Rather than do a way more crazy stop-gap like this (which actually works):

The below looks about like I expected it to. Thanks for coding it up and
trying it!

Agreed that it is more verbose than the original patch, but this seems
more self-documenting to me and looks narrow enough to backport to LTS
kernels or even move these helpers to common headers if they are found
to be more broadly useful. I have only a mild preference for this
solution over the initial one.

One more comment below.

> fs/nfs/localio.c | 6 +++---
> fs/nfs_common/nfslocalio.c | 8 +++----
> include/linux/nfslocalio.h | 52 ++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 59 insertions(+), 7 deletions(-)
>
> diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
> index 73dd07495440..fedc07254c00 100644
> --- a/fs/nfs/localio.c
> +++ b/fs/nfs/localio.c
> @@ -272,7 +272,7 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
>
> new = NULL;
> rcu_read_lock();
> - nf = rcu_dereference(*pnf);
> + nf = rcu_dereference_opaque(*pnf);
> if (!nf) {
> rcu_read_unlock();
> new = __nfs_local_open_fh(clp, cred, fh, nfl, mode);
> @@ -281,11 +281,11 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
> rcu_read_lock();
> /* try to swap in the pointer */
> spin_lock(&clp->cl_uuid.lock);
> - nf = rcu_dereference_protected(*pnf, 1);
> + nf = rcu_dereference_opaque_protected(*pnf, 1);
> if (!nf) {
> nf = new;
> new = NULL;
> - rcu_assign_pointer(*pnf, nf);
> + rcu_assign_opaque_pointer(*pnf, nf);
> }
> spin_unlock(&clp->cl_uuid.lock);
> }
> diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
> index 6a0bdea6d644..213862ceb8bb 100644
> --- a/fs/nfs_common/nfslocalio.c
> +++ b/fs/nfs_common/nfslocalio.c
> @@ -285,14 +285,14 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl)
> return;
> }
>
> - ro_nf = rcu_access_pointer(nfl->ro_file);
> - rw_nf = rcu_access_pointer(nfl->rw_file);
> + ro_nf = rcu_access_opaque(nfl->ro_file);
> + rw_nf = rcu_access_opaque(nfl->rw_file);
> if (ro_nf || rw_nf) {
> spin_lock(&nfs_uuid->lock);
> if (ro_nf)
> - ro_nf = rcu_dereference_protected(xchg(&nfl->ro_file, NULL), 1);
> + ro_nf = rcu_dereference_opaque_protected(xchg(&nfl->ro_file, NULL), 1);
> if (rw_nf)
> - rw_nf = rcu_dereference_protected(xchg(&nfl->rw_file, NULL), 1);
> + rw_nf = rcu_dereference_opaque_protected(xchg(&nfl->rw_file, NULL), 1);

I might combine rcu_dereference_opaque_protected and xchg into another
helper, since this rather verbose idiom appears twice.

> /* Remove nfl from nfs_uuid->files list */
> RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
> diff --git a/include/linux/nfslocalio.h b/include/linux/nfslocalio.h
> index 9aa8a43843d7..c6e86891d4b5 100644
> --- a/include/linux/nfslocalio.h
> +++ b/include/linux/nfslocalio.h
> @@ -15,6 +15,58 @@
> #include <linux/sunrpc/svcauth.h>
> #include <linux/nfs.h>
> #include <net/net_namespace.h>
> +#include <linux/rcupdate.h>
> +
> +/*
> + * RCU methods to allow fs/nfs_common and fs/nfs LOCALIO code to avoid
> + * dereferencing pointer to 'struct nfs_file' which is opaque outside fs/nfsd
> +*/
> +#define __rcu_access_opaque_pointer(p, local, space) \
> +({ \
> + typeof(p) local = (__force typeof(p))READ_ONCE(p); \
> + rcu_check_sparse(p, space); \
> + (__force __kernel typeof(p))(local); \
> +})
> +
> +#define rcu_access_opaque(p) __rcu_access_opaque_pointer((p), __UNIQUE_ID(rcu), __rcu)
> +
> +#define __rcu_dereference_opaque_protected(p, local, c, space) \
> +({ \
> + RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_opaque_protected() usage"); \
> + rcu_check_sparse(p, space); \
> + (__force __kernel typeof(p))(p); \
> +})
> +
> +#define rcu_dereference_opaque_protected(p, c) \
> + __rcu_dereference_opaque_protected((p), __UNIQUE_ID(rcu), (c), __rcu)
> +
> +#define __rcu_dereference_opaque_check(p, local, c, space) \
> +({ \
> + /* Dependency order vs. p above. */ \
> + typeof(p) local = (__force typeof(p))READ_ONCE(p); \
> + RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_opaque_check() usage"); \
> + rcu_check_sparse(p, space); \
> + (__force __kernel typeof(p))(local); \
> +})
> +
> +#define rcu_dereference_opaque_check(p, c) \
> + __rcu_dereference_opaque_check((p), __UNIQUE_ID(rcu), \
> + (c) || rcu_read_lock_held(), __rcu)
> +
> +#define rcu_dereference_opaque(p) rcu_dereference_opaque_check(p, 0)
> +
> +#define RCU_INITIALIZER_OPAQUE(v) (typeof((v)) __force __rcu)(v)
> +
> +#define rcu_assign_opaque_pointer(p, v) \
> +do { \
> + uintptr_t _r_a_p__v = (uintptr_t)(v); \
> + rcu_check_sparse(p, __rcu); \
> + \
> + if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL) \
> + WRITE_ONCE((p), (typeof(p))(_r_a_p__v)); \
> + else \
> + smp_store_release(&p, RCU_INITIALIZER_OPAQUE((typeof(p))_r_a_p__v)); \
> +} while (0)
>
> struct nfs_client;
> struct nfs_file_localio;

--
Chuck Lever