Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Trond Myklebust]

Hi Paul,

On Wed, 2018-10-31 at 23:32 +0000, Paul Burton wrote:
> (Copying SunRPC & net maintainers.)
> 
> Hi Guenter,
> 
> On Wed, Oct 31, 2018 at 03:02:53PM -0700, Guenter Roeck wrote:
> > The alternatives I can see are
> > - Do not use cmpxchg64() outside architecture code (ie drop its use
> > from
> >   the offending driver, and keep doing the same whenever the
> > problem comes
> >   up again).
> > or
> > - Introduce something like ARCH_HAS_CMPXCHG64 and use it to
> > determine
> >   if cmpxchg64 is supported or not.
> > 
> > Any preference ?
> 
> My preference would be option 1 - avoiding cmpxchg64() where possible
> in
> generic code. I wouldn't be opposed to the Kconfig option if there
> are
> cases where cmpxchg64() can really help performance though.
> 
> The last time I'm aware of this coming up the affected driver was
> modified to avoid cmpxchg64() [1].
> 
> In this particular case I have no idea why
> net/sunrpc/auth_gss/gss_krb5_seal.c is using cmpxchg64() at all. It's
> essentially reinventing atomic64_fetch_inc() which is already
> provided
> everywhere via CONFIG_GENERIC_ATOMIC64 & the spinlock approach. At
> least
> for atomic64_* functions the assumption that all access will be
> performed using those same functions seems somewhat reasonable.
> 
> So how does the below look? Trond?
> 

My one question (and the reason why I went with cmpxchg() in the first
place) would be about the overflow behaviour for atomic_fetch_inc() and
friends. I believe those functions should be OK on x86, so that when we
overflow the counter, it behaves like an unsigned value and wraps back
around.  Is that the case for all architectures?

i.e. are atomic_t/atomic64_t always guaranteed to behave like u32/u64
on increment?

I could not find any documentation that explicitly stated that they
should.

Cheers
  Trond

> Thanks,
>     Paul
> 
> [1] https://patchwork.ozlabs.org/cover/891284/
> 
> ---
> diff --git a/include/linux/sunrpc/gss_krb5.h
> b/include/linux/sunrpc/gss_krb5.h
> index 131424cefc6a..02c0412e368c 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)
>  
> diff --git a/net/sunrpc/auth_gss/gss_krb5_mech.c
> b/net/sunrpc/auth_gss/gss_krb5_mech.c
> index 7f0424dfa8f6..eab71fc7af3e 100644
> --- a/net/sunrpc/auth_gss/gss_krb5_mech.c
> +++ b/net/sunrpc/auth_gss/gss_krb5_mech.c
> @@ -274,6 +274,7 @@ get_key(const void *p, const void *end,
>  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));
> @@ -315,9 +316,10 @@ 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;
> @@ -607,6 +609,7 @@ 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));
> @@ -617,14 +620,15 @@ 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;
>  	}
> diff --git a/net/sunrpc/auth_gss/gss_krb5_seal.c
> b/net/sunrpc/auth_gss/gss_krb5_seal.c
> index b4adeb06660b..48fe4a591b54 100644
> --- a/net/sunrpc/auth_gss/gss_krb5_seal.c
> +++ b/net/sunrpc/auth_gss/gss_krb5_seal.c
> @@ -123,30 +123,6 @@ setup_token_v2(struct krb5_ctx *ctx, struct
> xdr_netobj *token)
>  	return krb5_hdr;
>  }
>  
> -u32
> -gss_seq_send_fetch_and_inc(struct krb5_ctx *ctx)
> -{
> -	u32 old, seq_send = READ_ONCE(ctx->seq_send);
> -
> -	do {
> -		old = seq_send;
> -		seq_send = cmpxchg(&ctx->seq_send, old, old + 1);
> -	} while (old != seq_send);
> -	return seq_send;
> -}
> -
> -u64
> -gss_seq_send64_fetch_and_inc(struct krb5_ctx *ctx)
> -{
> -	u64 old, seq_send = READ_ONCE(ctx->seq_send);
> -
> -	do {
> -		old = seq_send;
> -		seq_send = cmpxchg64(&ctx->seq_send64, old, old + 1);
> -	} while (old != seq_send);
> -	return seq_send;
> -}
> -
>  static u32
>  gss_get_mic_v1(struct krb5_ctx *ctx, struct xdr_buf *text,
>  		struct xdr_netobj *token)
> @@ -177,7 +153,7 @@ gss_get_mic_v1(struct krb5_ctx *ctx, struct
> xdr_buf *text,
>  
>  	memcpy(ptr + GSS_KRB5_TOK_HDR_LEN, md5cksum.data,
> md5cksum.len);
>  
> -	seq_send = gss_seq_send_fetch_and_inc(ctx);
> +	seq_send = atomic_fetch_inc(&ctx->seq_send);
>  
>  	if (krb5_make_seq_num(ctx, ctx->seq, ctx->initiate ? 0 : 0xff,
>  			      seq_send, ptr + GSS_KRB5_TOK_HDR_LEN, ptr
> + 8))
> @@ -205,7 +181,7 @@ gss_get_mic_v2(struct krb5_ctx *ctx, struct
> xdr_buf *text,
>  
>  	/* Set up the sequence number. Now 64-bits in clear
>  	 * text and w/o direction indicator */
> -	seq_send_be64 = cpu_to_be64(gss_seq_send64_fetch_and_inc(ctx));
> +	seq_send_be64 = cpu_to_be64(atomic64_fetch_inc(&ctx-
> >seq_send64));
>  	memcpy(krb5_hdr + 8, (char *) &seq_send_be64, 8);
>  
>  	if (ctx->initiate) {
> diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c
> b/net/sunrpc/auth_gss/gss_krb5_wrap.c
> index 962fa84e6db1..5cdde6cb703a 100644
> --- a/net/sunrpc/auth_gss/gss_krb5_wrap.c
> +++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
> @@ -228,7 +228,7 @@ gss_wrap_kerberos_v1(struct krb5_ctx *kctx, int
> offset,
>  
>  	memcpy(ptr + GSS_KRB5_TOK_HDR_LEN, md5cksum.data,
> md5cksum.len);
>  
> -	seq_send = gss_seq_send_fetch_and_inc(kctx);
> +	seq_send = atomic_fetch_inc(&kctx->seq_send);
>  
>  	/* XXX would probably be more efficient to compute checksum
>  	 * and encrypt at the same time: */
> @@ -475,7 +475,7 @@ gss_wrap_kerberos_v2(struct krb5_ctx *kctx, u32
> offset,
>  	*be16ptr++ = 0;
>  
>  	be64ptr = (__be64 *)be16ptr;
> -	*be64ptr = cpu_to_be64(gss_seq_send64_fetch_and_inc(kctx));
> +	*be64ptr = cpu_to_be64(atomic64_fetch_inc(&kctx->seq_send64));
>  
>  	err = (*kctx->gk5e->encrypt_v2)(kctx, offset, buf, pages);
>  	if (err)
-- 
Trond Myklebust
CTO, Hammerspace Inc
4300 El Camino Real, Suite 105
Los Altos, CA 94022
www.hammer.space