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, 10 insertions, 6 deletions

diff --git a/net/sunrpc/auth_gss/gss_krb5_mech.c b/net/sunrpc/auth_gss/gss_krb5_mech.c
index 7bb2514aadd9d8..71cb29dc86c283 100644
--- a/net/sunrpc/auth_gss/gss_krb5_mech.c
+++ b/net/sunrpc/auth_gss/gss_krb5_mech.c
@@ -275,6 +275,7 @@ out_err:
 static int
 gss_import_v1_context(const void *p, const void *end, struct krb5_ctx *ctx)
 {
+	u32 seq_send;
 	int tmp;
 
 	p = simple_get_bytes(p, end, &ctx->initiate, sizeof(ctx->initiate));
@@ -316,9 +317,10 @@ gss_import_v1_context(const void *p, const void *end, struct krb5_ctx *ctx)
 	p = simple_get_bytes(p, end, &ctx->endtime, sizeof(ctx->endtime));
 	if (IS_ERR(p))
 		goto out_err;
-	p = simple_get_bytes(p, end, &ctx->seq_send, sizeof(ctx->seq_send));
+	p = simple_get_bytes(p, end, &seq_send, sizeof(seq_send));
 	if (IS_ERR(p))
 		goto out_err;
+	atomic_set(&ctx->seq_send, seq_send);
 	p = simple_get_netobj(p, end, &ctx->mech_used);
 	if (IS_ERR(p))
 		goto out_err;
@@ -610,6 +612,7 @@ static int
 gss_import_v2_context(const void *p, const void *end, struct krb5_ctx *ctx,
 		gfp_t gfp_mask)
 {
+	u64 seq_send64;
 	int keylen;
 
 	p = simple_get_bytes(p, end, &ctx->flags, sizeof(ctx->flags));
@@ -620,14 +623,15 @@ gss_import_v2_context(const void *p, const void *end, struct krb5_ctx *ctx,
 	p = simple_get_bytes(p, end, &ctx->endtime, sizeof(ctx->endtime));
 	if (IS_ERR(p))
 		goto out_err;
-	p = simple_get_bytes(p, end, &ctx->seq_send64, sizeof(ctx->seq_send64));
+	p = simple_get_bytes(p, end, &seq_send64, sizeof(seq_send64));
 	if (IS_ERR(p))
 		goto out_err;
+	atomic64_set(&ctx->seq_send64, seq_send64);
 	/* set seq_send for use by "older" enctypes */
-	ctx->seq_send = ctx->seq_send64;
-	if (ctx->seq_send64 != ctx->seq_send) {
-		dprintk("%s: seq_send64 %lx, seq_send %x overflow?\n", __func__,
-			(unsigned long)ctx->seq_send64, ctx->seq_send);
+	atomic_set(&ctx->seq_send, seq_send64);
+	if (seq_send64 != atomic_read(&ctx->seq_send)) {
+		dprintk("%s: seq_send64 %llx, seq_send %x overflow?\n", __func__,
+			seq_send64, atomic_read(&ctx->seq_send));
 		p = ERR_PTR(-EINVAL);
 		goto out_err;
 	}