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

Pali Rohár
2025-05-18T10:46:51Z

On Friday 16 May 2025 11:33:20 Chuck Lever wrote:
> 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!

This really looks nice. Thanks.

Would you send it as a regular patch to the list?

> 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