path: root/ipc
diff options
authorNeilBrown <neilb@suse.com>2019-04-02 10:07:45 +1100
committerDavid S. Miller <davem@davemloft.net>2019-04-07 19:12:12 -0700
commit8f0db018006a421956965e1149234c4e8db718ee (patch)
tree40a6c226d3dc13cebf65017a23061e50007c0ef0 /ipc
parentff302db965b57c141297911ea647d36d11fedfbe (diff)
rhashtable: use bit_spin_locks to protect hash bucket.
This patch changes rhashtables to use a bit_spin_lock on BIT(1) of the bucket pointer to lock the hash chain for that bucket. The benefits of a bit spin_lock are: - no need to allocate a separate array of locks. - no need to have a configuration option to guide the choice of the size of this array - locking cost is often a single test-and-set in a cache line that will have to be loaded anyway. When inserting at, or removing from, the head of the chain, the unlock is free - writing the new address in the bucket head implicitly clears the lock bit. For __rhashtable_insert_fast() we ensure this always happens when adding a new key. - even when lockings costs 2 updates (lock and unlock), they are in a cacheline that needs to be read anyway. The cost of using a bit spin_lock is a little bit of code complexity, which I think is quite manageable. Bit spin_locks are sometimes inappropriate because they are not fair - if multiple CPUs repeatedly contend of the same lock, one CPU can easily be starved. This is not a credible situation with rhashtable. Multiple CPUs may want to repeatedly add or remove objects, but they will typically do so at different buckets, so they will attempt to acquire different locks. As we have more bit-locks than we previously had spinlocks (by at least a factor of two) we can expect slightly less contention to go with the slightly better cache behavior and reduced memory consumption. To enhance type checking, a new struct is introduced to represent the pointer plus lock-bit that is stored in the bucket-table. This is "struct rhash_lock_head" and is empty. A pointer to this needs to be cast to either an unsigned lock, or a "struct rhash_head *" to be useful. Variables of this type are most often called "bkt". Previously "pprev" would sometimes point to a bucket, and sometimes a ->next pointer in an rhash_head. As these are now different types, pprev is NULL when it would have pointed to the bucket. In that case, 'blk' is used, together with correct locking protocol. Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'ipc')
1 files changed, 0 insertions, 1 deletions
diff --git a/ipc/util.c b/ipc/util.c
index 0af05752969f..095274a871f8 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -101,7 +101,6 @@ static const struct rhashtable_params ipc_kht_params = {
.head_offset = offsetof(struct kern_ipc_perm, khtnode),
.key_offset = offsetof(struct kern_ipc_perm, key),
.key_len = FIELD_SIZEOF(struct kern_ipc_perm, key),
- .locks_mul = 1,
.automatic_shrinking = true,

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