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Subject[PATCH 1/3] jbd2,rcu: correctly use RCU
Date
In read site, we need to use local_ptr = rcu_dereference(),
and then use this local_ptr to read the content.

In update site, we should assign/publish the new object/pointer after
the content of the new object/pointer is fully initialized,
and we can't not touch the object after the pointer assignment.
rcu_assign_pointer() is need for the assignement.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
fs/jbd2/journal.c | 25 +++++++++++++------------
1 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 9267291..1c45b6a 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2439,7 +2439,7 @@ struct devname_cache {
char devname[BDEVNAME_SIZE];
};
#define CACHE_SIZE_BITS 6
-static struct devname_cache *devcache[1 << CACHE_SIZE_BITS];
+static struct devname_cache __rcu *devcache[1 << CACHE_SIZE_BITS];
static DEFINE_SPINLOCK(devname_cache_lock);

const char *jbd2_dev_to_name(dev_t device)
@@ -2447,24 +2447,25 @@ const char *jbd2_dev_to_name(dev_t device)
int i = hash_32(device, CACHE_SIZE_BITS);
char *ret;
struct block_device *bd;
- static struct devname_cache *new_dev;
+ static struct devname_cache *cache;

rcu_read_lock();
- if (devcache[i] && devcache[i]->device == device) {
- ret = devcache[i]->devname;
+ cache = rcu_dereference(devcache[i]);
+ if (cache && cache->device == device) {
+ ret = cache->devname;
rcu_read_unlock();
return ret;
}
rcu_read_unlock();

- new_dev = kmalloc(sizeof(struct devname_cache), GFP_KERNEL);
- if (!new_dev)
+ cache = kmalloc(sizeof(struct devname_cache), GFP_KERNEL);
+ if (!cache)
return "NODEV-ALLOCFAILURE"; /* Something non-NULL */
bd = bdget(device);
spin_lock(&devname_cache_lock);
if (devcache[i]) {
if (devcache[i]->device == device) {
- kfree(new_dev);
+ kfree(cache);
bdput(bd);
ret = devcache[i]->devname;
spin_unlock(&devname_cache_lock);
@@ -2472,14 +2473,14 @@ const char *jbd2_dev_to_name(dev_t device)
}
kfree_rcu(devcache[i], rcu);
}
- devcache[i] = new_dev;
- devcache[i]->device = device;
+ cache->device = device;
if (bd) {
- bdevname(bd, devcache[i]->devname);
+ bdevname(bd, cache->devname);
bdput(bd);
} else
- __bdevname(device, devcache[i]->devname);
- ret = devcache[i]->devname;
+ __bdevname(device, cache->devname);
+ rcu_assign_pointer(devcache[i], cache);
+ ret = cache->devname;
spin_unlock(&devname_cache_lock);
return ret;
}
--
1.7.4.4


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