Messages in this thread | | | Date | Wed, 20 Mar 2019 13:40:21 +0800 | From | Herbert Xu <> | Subject | Re: [PATCH 2/3] rhashtable: don't hold lock on first table throughout insertion. |
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On Fri, Mar 15, 2019 at 05:46:37PM +1100, NeilBrown wrote: > > From: NeilBrown <neilb@suse.com> > Date: Mon, 25 Jun 2018 08:09:19 +1000 > Subject: [PATCH] rhashtable: don't hold lock on first table throughout > insertion. > > rhashtable_try_insert() currently holds a lock on the bucket in > the first table, while also locking buckets in subsequent tables. > This is unnecessary and looks like a hold-over from some earlier > version of the implementation. > > As insert and remove always lock a bucket in each table in turn, and > as insert only inserts in the final table, there cannot be any races > that are not covered by simply locking a bucket in each table in turn. > > When an insert call reaches that last table it can be sure that there > is no matchinf entry in any other table as it has searched them all, and > insertion never happens anywhere but in the last table. The fact that > code tests for the existence of future_tbl while holding a lock on > the relevant bucket ensures that two threads inserting the same key > will make compatible decisions about which is the "last" table. > > This simplifies the code and allows the ->rehash field to be > discarded. > > We still need a way to ensure that a dead bucket_table is never > re-linked by rhashtable_walk_stop(). This can be achieved by calling > call_rcu() inside the locked region, and checking with > rcu_head_after_call_rcu() in rhashtable_walk_stop() to see if the > bucket table is empty and dead. > > Signed-off-by: NeilBrown <neilb@suse.com> > --- > include/linux/rhashtable.h | 13 ------------ > lib/rhashtable.c | 52 ++++++++++++++-------------------------------- > 2 files changed, 16 insertions(+), 49 deletions(-)
Acked-by: Herbert Xu <herbert@gondor.apana.org.au> -- Email: Herbert Xu <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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