From 84e710da2a1dfacfc87f604869a4d22df91ce6cd Mon Sep 17 00:00:00 2001 From: Al Viro Date: Fri, 15 Apr 2016 00:58:55 -0400 Subject: 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 --- Documentation/filesystems/porting | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'Documentation/filesystems') diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting index 57bb3754a027..8810e2367fe6 100644 --- a/Documentation/filesystems/porting +++ b/Documentation/filesystems/porting @@ -531,3 +531,11 @@ in your dentry operations instead. dentry might be yet to be attached to inode, so do _not_ use its ->d_inode in the instances. Rationale: !@#!@# security_d_instantiate() needs to be called before we attach dentry to inode. +-- +[mandatory] + symlinks are no longer the only inodes that do *not* have i_bdev/i_cdev/ + i_pipe/i_link union zeroed out at inode eviction. As the result, you can't + assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that + it's a symlink. Checking ->i_mode is really needed now. In-tree we had + to fix shmem_destroy_callback() that used to take that kind of shortcut; + watch out, since that shortcut is no longer valid. -- cgit v1.2.3