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authorAl Viro <viro@zeniv.linux.org.uk>2016-04-15 07:58:55 +0300
committerAl Viro <viro@zeniv.linux.org.uk>2016-05-03 02:49:26 +0300
commit84e710da2a1dfacfc87f604869a4d22df91ce6cd (patch)
tree37625d5ef48c43a17c6da2f5a80b107da5524957 /Documentation/filesystems/porting
parent85c7f81041d57cfe9dc97f4680d5586b54534a39 (diff)
downloadlinux-84e710da2a1dfacfc87f604869a4d22df91ce6cd.tar.xz
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 'Documentation/filesystems/porting')
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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.