From a890643958f03aaa344290700093b280cb606c28 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Alex=20Benn=C3=A9e?= Date: Fri, 30 Sep 2016 22:31:00 +0100 Subject: [PATCH] util/qht: atomically set b->hashes MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit ThreadSanitizer detects a possible race between reading/writing the hashes. The ordering semantics are already documented for QHT however for true C11 compliance we should use relaxed atomic primitives for accesses that are done across threads. On x86 this slightly changes to the code to not do a load/compare in a single instruction leading to a slight performance degradation. Running 'taskset -c 0 tests/qht-bench -n 1 -d 10' (i.e. all lookups) 10 times, we get: before the patch: $ ./mean.pl 34.04 34.24 34.38 34.25 34.18 34.51 34.46 34.44 34.29 34.08 34.287 +- 0.160072900059109 after: $ ./mean.pl 33.94 34.00 33.52 33.46 33.55 33.71 34.27 34.06 34.28 34.58 33.937 +- 0.374731014640279 Signed-off-by: Alex Bennée Reviewed-by: Emilio G. Cota Message-Id: <20160930213106.20186-10-alex.bennee@linaro.org> Signed-off-by: Paolo Bonzini --- util/qht.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/util/qht.c b/util/qht.c index 16a8d7950e..571639d30a 100644 --- a/util/qht.c +++ b/util/qht.c @@ -379,7 +379,7 @@ static void qht_bucket_reset__locked(struct qht_bucket *head) if (b->pointers[i] == NULL) { goto done; } - b->hashes[i] = 0; + atomic_set(&b->hashes[i], 0); atomic_set(&b->pointers[i], NULL); } b = b->next; @@ -444,7 +444,7 @@ void *qht_do_lookup(struct qht_bucket *head, qht_lookup_func_t func, do { for (i = 0; i < QHT_BUCKET_ENTRIES; i++) { - if (b->hashes[i] == hash) { + if (atomic_read(&b->hashes[i]) == hash) { /* The pointer is dereferenced before seqlock_read_retry, * so (unlike qht_insert__locked) we need to use * atomic_rcu_read here. @@ -538,8 +538,8 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map, if (new) { atomic_rcu_set(&prev->next, b); } - b->hashes[i] = hash; /* smp_wmb() implicit in seqlock_write_begin. */ + atomic_set(&b->hashes[i], hash); atomic_set(&b->pointers[i], p); seqlock_write_end(&head->sequence); return true; @@ -607,10 +607,10 @@ qht_entry_move(struct qht_bucket *to, int i, struct qht_bucket *from, int j) qht_debug_assert(to->pointers[i]); qht_debug_assert(from->pointers[j]); - to->hashes[i] = from->hashes[j]; + atomic_set(&to->hashes[i], from->hashes[j]); atomic_set(&to->pointers[i], from->pointers[j]); - from->hashes[j] = 0; + atomic_set(&from->hashes[j], 0); atomic_set(&from->pointers[j], NULL); }