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author | Al Viro <viro@zeniv.linux.org.uk> | 2016-04-15 00:58:55 -0400 |
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committer | Al Viro <viro@zeniv.linux.org.uk> | 2016-05-02 19:49:26 -0400 |
commit | 84e710da2a1dfacfc87f604869a4d22df91ce6cd (patch) | |
tree | 37625d5ef48c43a17c6da2f5a80b107da5524957 /include/linux/fs.h | |
parent | 85c7f81041d57cfe9dc97f4680d5586b54534a39 (diff) | |
download | op-kernel-dev-84e710da2a1dfacfc87f604869a4d22df91ce6cd.zip op-kernel-dev-84e710da2a1dfacfc87f604869a4d22df91ce6cd.tar.gz |
parallel lookups machinery, part 2
We'll need to verify that there's neither a hashed nor in-lookup
dentry with desired parent/name before adding to in-lookup set.
One possible solution would be to hold the parent's ->d_lock through
both checks, but while the in-lookup set is relatively small at any
time, dcache is not. And holding the parent's ->d_lock through
something like __d_lookup_rcu() would suck too badly.
So we leave the parent's ->d_lock alone, which means that we watch
out for the following scenario:
* we verify that there's no hashed match
* existing in-lookup match gets hashed by another process
* we verify that there's no in-lookup matches and decide
that everything's fine.
Solution: per-directory kinda-sorta seqlock, bumped around the times
we hash something that used to be in-lookup or move (and hash)
something in place of in-lookup. Then the above would turn into
* read the counter
* do dcache lookup
* if no matches found, check for in-lookup matches
* if there had been none of those either, check if the
counter has changed; repeat if it has.
The "kinda-sorta" part is due to the fact that we don't have much spare
space in inode. There is a spare word (shared with i_bdev/i_cdev/i_pipe),
so the counter part is not a problem, but spinlock is a different story.
We could use the parent's ->d_lock, and it would be less painful in
terms of contention, for __d_add() it would be rather inconvenient to
grab; we could do that (using lock_parent()), but...
Fortunately, we can get serialization on the counter itself, and it
might be a good idea in general; we can use cmpxchg() in a loop to
get from even to odd and smp_store_release() from odd to even.
This commit adds the counter and updating logics; the readers will be
added in the next commit.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'include/linux/fs.h')
-rw-r--r-- | include/linux/fs.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/include/linux/fs.h b/include/linux/fs.h index 6d0fa91..00cecc5 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -684,6 +684,7 @@ struct inode { struct block_device *i_bdev; struct cdev *i_cdev; char *i_link; + unsigned i_dir_seq; }; __u32 i_generation; |