On Dec 10, 2012, at 3:02 PM, "Myklebust, Trond" <Trond.Myklebust@xxxxxxxxxx> wrote:
> On Fri, 2012-11-30 at 13:09 -0500, andros@xxxxxxxxxx wrote:
>> From: Andy Adamson <andros@xxxxxxxxxx>
>>
>> Signed-off-by: Andy Adamson <andros@xxxxxxxxxx>
>> ---
>> A cleanup patch.
>>
>> net/sunrpc/auth.c | 1 +
>> net/sunrpc/auth_gss/auth_gss.c | 1 +
>> 2 files changed, 2 insertions(+), 0 deletions(-)
>>
>> diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
>> index b5c067b..8b76753 100644
>> --- a/net/sunrpc/auth.c
>> +++ b/net/sunrpc/auth.c
>> @@ -225,6 +225,7 @@ rpcauth_unhash_cred_locked(struct rpc_cred *cred)
>> hlist_del_rcu(&cred->cr_hash);
>> smp_mb__before_clear_bit();
>> clear_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags);
>> + smp_mb__after_clear_bit();
>> }
>>
>> static int
>> diff --git a/net/sunrpc/auth_gss/auth_gss.c b/net/sunrpc/auth_gss/auth_gss.c
>> index 909dc0c..92a7d6d 100644
>> --- a/net/sunrpc/auth_gss/auth_gss.c
>> +++ b/net/sunrpc/auth_gss/auth_gss.c
>> @@ -126,6 +126,7 @@ gss_cred_set_ctx(struct rpc_cred *cred, struct gss_cl_ctx *ctx)
>> set_bit(RPCAUTH_CRED_UPTODATE, &cred->cr_flags);
>> smp_mb__before_clear_bit();
>> clear_bit(RPCAUTH_CRED_NEW, &cred->cr_flags);
>> + smp_mb__after_clear_bit();
>> }
>>
>> static const void *
>
> Why are these needed?
AFAICS the documentation says to either call both smp_mb__before_clear_bit and smp_mp__after_clear_bit to ensure that memory operations before and after the clear_bit call are strongly ordered, or don't call either. Since there can be multiple threads looking at a gss_cred cr_flags, it seems to me that we want the memory barriers to get the correct value of the bit.
-->Andy
from Documentation/atomic_ops.txt:
If explicit memory barriers are required around clear_bit() (which
does not return a value, and thus does not need to provide memory
barrier semantics), two interfaces are provided:
void smp_mb__before_clear_bit(void);
void smp_mb__after_clear_bit(void);
They are used as follows, and are akin to their atomic_t operation
brothers:
/* All memory operations before this call will
* be globally visible before the clear_bit().
*/
smp_mb__before_clear_bit();
clear_bit( ... );
/* The clear_bit() will be visible before all
* subsequent memory operations.
*/
smp_mb__after_clear_bit();
>
> --
> Trond Myklebust
> Linux NFS client maintainer
>
> NetApp
> Trond.Myklebust@xxxxxxxxxx
> www.netapp.com
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