SUNRPC: Use atomic(64)_t for seq_send(64)

The seq_send & seq_send64 fields in struct krb5_ctx are used as
atomically incrementing counters. This is implemented using cmpxchg() &
cmpxchg64() to implement what amount to custom versions of
atomic_fetch_inc() & atomic64_fetch_inc().

Besides the duplication, using cmpxchg64() has another major drawback in
that some 32 bit architectures don't provide it. As such commit
571ed1fd2390 ("SUNRPC: Replace krb5_seq_lock with a lockless scheme")
resulted in build failures for some architectures.

Change seq_send to be an atomic_t and seq_send64 to be an atomic64_t,
then use atomic(64)_* functions to manipulate the values. The atomic64_t
type & associated functions are provided even on architectures which
lack real 64 bit atomic memory access via CONFIG_GENERIC_ATOMIC64 which
uses spinlocks to serialize access. This fixes the build failures for
architectures lacking cmpxchg64().

A potential alternative that was raised would be to provide cmpxchg64()
on the 32 bit architectures that currently lack it, using spinlocks.
However this would provide a version of cmpxchg64() with semantics a
little different to the implementations on architectures with real 64
bit atomics - the spinlock-based implementation would only work if all
access to the memory used with cmpxchg64() is *always* performed using
cmpxchg64(). That is not currently a requirement for users of
cmpxchg64(), and making it one seems questionable. As such avoiding
cmpxchg64() outside of architecture-specific code seems best,
particularly in cases where atomic64_t seems like a better fit anyway.

The CONFIG_GENERIC_ATOMIC64 implementation of atomic64_* functions will
use spinlocks & so faces the same issue, but with the key difference
that the memory backing an atomic64_t ought to always be accessed via
the atomic64_* functions anyway making the issue moot.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 571ed1fd2390 ("SUNRPC: Replace krb5_seq_lock with a lockless scheme")
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: linux-nfs@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

1 files changed, 2 insertions, 5 deletions

diff --git a/include/linux/sunrpc/gss_krb5.h b/include/linux/sunrpc/gss_krb5.h
index 69f749afa617..4162de72e95c 100644
--- a/include/linux/sunrpc/gss_krb5.h
+++ b/include/linux/sunrpc/gss_krb5.h
@@ -107,8 +107,8 @@ struct krb5_ctx {
 	u8			Ksess[GSS_KRB5_MAX_KEYLEN]; /* session key */
 	u8			cksum[GSS_KRB5_MAX_KEYLEN];
 	s32			endtime;
-	u32			seq_send;
-	u64			seq_send64;
+	atomic_t		seq_send;
+	atomic64_t		seq_send64;
 	struct xdr_netobj	mech_used;
 	u8			initiator_sign[GSS_KRB5_MAX_KEYLEN];
 	u8			acceptor_sign[GSS_KRB5_MAX_KEYLEN];
@@ -118,9 +118,6 @@ struct krb5_ctx {
 	u8			acceptor_integ[GSS_KRB5_MAX_KEYLEN];
 };
 
-extern u32 gss_seq_send_fetch_and_inc(struct krb5_ctx *ctx);
-extern u64 gss_seq_send64_fetch_and_inc(struct krb5_ctx *ctx);
-
 /* The length of the Kerberos GSS token header */
 #define GSS_KRB5_TOK_HDR_LEN	(16)