Comments? Please :-?
ok :) see inline.
Somewhere in among the comments (thanks), I realised that I was only
closing half the race. I had tried to make sure there were no stray
references to any dentries, but there is still the inode which is
being iput which can cause problem.
The following patch takes a totally different approach, is based on an
idea from Jan Kara, and is much less intrusive.
We:
- keep track of "who" is calling prune_dcache, and when a filesystem
is being unmounted (s_root == NULL) we only allow the unmount thread
to prune dentries.
- keep track of how many dentries are in the process of having
dentry_iput called on them for pruning
- don't allow umount to proceed until that count hits zero
- bias the count this way and that to make sure we get a wake_up at
the right time
- reuse 's_wait_unfrozen' to wait on the iput to complete.
Again, I'm very keen on feedback. This race is very hard to trigger,
so code review is the only real way to evaluate that patch.
hmm... It's not that very hard. I suppose no one ever tried except us.
We have a test which does mounts/umounts and lots of other activity
which makes memory pressure. AFAIR, it takes us ~3 hours to trigger this
bug on SMP.
In general your patch is still does what mine do, so I will be happy if
any of this is commited mainstream. In future, please, keep the
reference to original authors, this will also make sure that I'm on CC
if something goes wrong.
Thanks,
Kiril
Signed-off-by: Neil Brown <[email protected]>
### Diffstat output
./fs/dcache.c | 17 +++++++++++++----
./fs/super.c | 11 +++++++++++
./include/linux/fs.h | 2 ++
3 files changed, 26 insertions(+), 4 deletions(-)
diff ./fs/dcache.c~current~ ./fs/dcache.c
--- ./fs/dcache.c~current~ 2006-03-06 16:54:59.000000000 +1100
+++ ./fs/dcache.c 2006-03-06 16:55:33.000000000 +1100
@@ -366,6 +366,7 @@ static inline void prune_one_dentry(stru
{
struct dentry * parent;
+ dentry->d_sb->s_pending_iputs ++;
__d_drop(dentry);
list_del(&dentry->d_u.d_child);
dentry_stat.nr_dentry--; /* For d_free, below */
@@ -375,6 +376,9 @@ static inline void prune_one_dentry(stru
if (parent != dentry)
dput(parent);
spin_lock(&dcache_lock);
+ dentry->d_sb->s_pending_iputs --;
+ if (dentry->d_sb->s_pending_iputs < 0)
+ wake_up(&dentry->d_sb->s_wait_unfrozen);
}
/**
@@ -390,7 +394,7 @@ static inline void prune_one_dentry(stru
* all the dentries are in use.
*/
-static void prune_dcache(int count)
+static void prune_dcache(int count, struct dentry *parent)
{
spin_lock(&dcache_lock);
for (; count ; count--) {
@@ -407,6 +411,11 @@ static void prune_dcache(int count)
dentry_stat.nr_unused--;
dentry = list_entry(tmp, struct dentry, d_lru);
+ if (dentry->d_sb->s_root == NULL &&
+ (parent == NULL ||
+ parent->d_sb != dentry->d_sb))
+ continue;
<<<<
- we select some dentries in select_parent and then try to prune N
dentries. But this can be other N dentries :/ It's not the problem with
your code, but... adding 'parent' arg to prune_dcache() makes me feel
the same as with 'found' arg: you don't use it actually right way. You
don't prune only its children, you see? It is better to pass 'sb' arg then.
+
spin_lock(&dentry->d_lock);
/*
* We found an inuse dentry which was not removed from
@@ -635,7 +644,7 @@ void shrink_dcache_parent(struct dentry
int found;
while ((found = select_parent(parent)) != 0)
- prune_dcache(found);
+ prune_dcache(found, parent);
}
/**
@@ -673,7 +682,7 @@ void shrink_dcache_anon(struct hlist_hea
}
}
spin_unlock(&dcache_lock);
- prune_dcache(found);
+ prune_dcache(found, NULL);
} while(found);
}
@@ -694,7 +703,7 @@ static int shrink_dcache_memory(int nr,
if (nr) {
if (!(gfp_mask & __GFP_FS))
return -1;
- prune_dcache(nr);
+ prune_dcache(nr, NULL);
}
return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
}
diff ./fs/super.c~current~ ./fs/super.c
--- ./fs/super.c~current~ 2006-03-06 16:54:59.000000000 +1100
+++ ./fs/super.c 2006-03-06 16:57:19.000000000 +1100
@@ -230,7 +230,18 @@ void generic_shutdown_super(struct super
struct super_operations *sop = sb->s_op;
if (root) {
+ spin_lock(&dcache_lock);
+ /* disable stray dputs */
sb->s_root = NULL;
+
+ /* trigger a wake_up */
+ sb->s_pending_iputs --;
+ spin_unlock(&dcache_lock);
+ wait_event(sb->s_wait_unfrozen,
+ sb->s_pending_iputs < 0);
+ /* avoid further wakeups */
+ sb->s_pending_iputs = 65000;
<<< its ulgy... :( why don't you do wait after shrink's and dput(root)
below?
+
shrink_dcache_parent(root);
shrink_dcache_anon(&sb->s_anon);
dput(root);
diff ./include/linux/fs.h~current~ ./include/linux/fs.h
--- ./include/linux/fs.h~current~ 2006-03-06 16:54:59.000000000 +1100
+++ ./include/linux/fs.h 2006-03-06 12:49:55.000000000 +1100
@@ -833,6 +833,8 @@ struct super_block {
struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
struct list_head s_files;
+ int s_pending_iputs;
+
struct block_device *s_bdev;
struct list_head s_instances;
struct quota_info s_dquot; /* Diskquota specific options */
-
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