Re: [PATCH 3/8] rhashtable: use cmpxchg() to protect ->future_tbl.

[NeilBrown]

On Sat, May 05 2018, Herbert Xu wrote:

> On Fri, May 04, 2018 at 01:54:14PM +1000, NeilBrown wrote:
>> Rather than borrowing one of the bucket locks to
>> protect ->future_tbl updates, use cmpxchg().
>> This gives more freedom to change how bucket locking
>> is implemented.
>> 
>> Signed-off-by: NeilBrown 
>
> This looks nice.
>
>> -spin_unlock_bh(old_tbl->locks);
>> +rcu_assign_pointer(tmp, new_tbl);
>
> Do we need this barrier since cmpxchg is supposed to provide memory
> barrier semantics?

It's hard to find documentation even for what cmpxchg() is meant do, let
alone what barriers is provides, but there does seem to be something
hidden in Documentation/core-api/atomic_ops.rst which suggests full
barrier semantics if the comparison succeeds.  I'll replace the
rcu_assign_pointer with a comment saying why it isn't needed.

Thanks,
NeilBrown

>
>> +if (cmpxchg(&old_tbl->future_tbl, NULL, tmp) != NULL)
>> +return -EEXIST;
>
> Thanks,
> -- 
> Email: Herbert Xu 
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt


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Re: [PATCH 3/8] rhashtable: use cmpxchg() to protect ->future_tbl.

[Herbert Xu]

On Fri, May 04, 2018 at 01:54:14PM +1000, NeilBrown wrote:
> Rather than borrowing one of the bucket locks to
> protect ->future_tbl updates, use cmpxchg().
> This gives more freedom to change how bucket locking
> is implemented.
> 
> Signed-off-by: NeilBrown 

This looks nice.

> - spin_unlock_bh(old_tbl->locks);
> + rcu_assign_pointer(tmp, new_tbl);

Do we need this barrier since cmpxchg is supposed to provide memory
barrier semantics?

> + if (cmpxchg(&old_tbl->future_tbl, NULL, tmp) != NULL)
> + return -EEXIST;

Thanks,
-- 
Email: Herbert Xu 
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

[PATCH 3/8] rhashtable: use cmpxchg() to protect ->future_tbl.

[NeilBrown]

Rather than borrowing one of the bucket locks to
protect ->future_tbl updates, use cmpxchg().
This gives more freedom to change how bucket locking
is implemented.

Signed-off-by: NeilBrown 
---
 lib/rhashtable.c |   17 ++---
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 4a3f94e8e8a6..b73afe1dec7e 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -298,21 +298,16 @@ static int rhashtable_rehash_attach(struct rhashtable *ht,
struct bucket_table *old_tbl,
struct bucket_table *new_tbl)
 {
-   /* Protect future_tbl using the first bucket lock. */
-   spin_lock_bh(old_tbl->locks);
-
-   /* Did somebody beat us to it? */
-   if (rcu_access_pointer(old_tbl->future_tbl)) {
-   spin_unlock_bh(old_tbl->locks);
-   return -EEXIST;
-   }
-
/* Make insertions go into the new, empty table right away. Deletions
 * and lookups will be attempted in both tables until we synchronize.
+* The use of 'tmp' is simply to ensure we get the required memory
+* barriers before the cmpxchg().
 */
-   rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
+   struct bucket_table *tmp;
 
-   spin_unlock_bh(old_tbl->locks);
+   rcu_assign_pointer(tmp, new_tbl);
+   if (cmpxchg(&old_tbl->future_tbl, NULL, tmp) != NULL)
+   return -EEXIST;
 
return 0;
 }