[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] [PULL 17/39] util/qht: atomically set b->hashes
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PULL 17/39] util/qht: atomically set b->hashes |
Date: |
Fri, 7 Oct 2016 18:57:41 +0200 |
From: Alex Bennée <address@hidden>
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 <address@hidden>
Reviewed-by: Emilio G. Cota <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
util/qht.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/util/qht.c b/util/qht.c
index 16a8d79..571639d 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);
}
--
2.7.4
- [Qemu-devel] [PULL 07/39] intc/lm32_pic: implement InterruptStatsProvider interface, (continued)
- [Qemu-devel] [PULL 07/39] intc/lm32_pic: implement InterruptStatsProvider interface, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 08/39] intc: make HMP 'info irq' and 'info pic' commands use InterruptStatsProvider interface, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 09/39] intc: make HMP 'info irq' and 'info pic' commands available on all targets, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 10/39] atomic.h: fix __SANITIZE_THREAD__ build, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 11/39] atomic.h: comment on use of atomic_read/set, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 12/39] tcg/optimize: move default return out of if statement, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 13/39] seqlock: use atomic writes for the sequence, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 14/39] qom/object: update class cache atomically, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 15/39] qom/cpu: atomically clear the tb_jmp_cache, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 16/39] cpu: atomically modify cpu->exit_request, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 17/39] util/qht: atomically set b->hashes,
Paolo Bonzini <=
- [Qemu-devel] [PULL 18/39] linux-user/syscall: extend lock around cpu-list, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 19/39] qga/command: use QEMU atomic primitives, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 20/39] .travis.yml: add gcc sanitizer build, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 22/39] char: use a fixed idx for child muxed chr, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 21/39] i8259: give ISA device when registering ISA ioports, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 23/39] char: update read handler in all cases, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 25/39] qemu_kill_report: Report PID name too, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 24/39] util: Introduce qemu_get_pid_name, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 26/39] qemu-nbd: Shrink image size by specified offset, Paolo Bonzini, 2016/10/07
- [Qemu-devel] [PULL 27/39] qht: simplify qht_reset_size, Paolo Bonzini, 2016/10/07