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fs: brlock vfsmount_lock

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fs: brlock vfsmount_lock

Use a brlock for the vfsmount lock. It must be taken for write whenever
modifying the mount hash or associated fields, and may be taken for read when
performing mount hash lookups.

A new lock is added for the mnt-id allocator, so it doesn't need to take
the heavy vfsmount write-lock.

The number of atomics should remain the same for fastpath rlock cases, though
code would be slightly slower due to per-cpu access. Scalability is not not be
much improved in common cases yet, due to other locks (ie. dcache_lock) getting
in the way. However path lookups crossing mountpoints should be one case where
scalability is improved (currently requiring the global lock).

The slowpath is slower due to use of brlock. On a 64 core, 64 socket, 32 node
Altix system (high latency to remote nodes), a simple umount microbenchmark
(mount --bind mnt mnt2 ; umount mnt2 loop 1000 times), before this patch it
took 6.8s, afterwards took 7.1s, about 5% slower.

Cc: Al Viro <[email protected]>
Signed-off-by: Nick Piggin <[email protected]>
Signed-off-by: Al Viro <[email protected]>
This commit is contained in:
Nick Piggin 2010-08-18 04:37:39 +10:00 • committed by Al Viro
commit 99b7db7b8f
5 changed files with 134 additions and 77 deletions
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11
fs/dcache.c
View file

@ -1935,7 +1935,7 @@ static int prepend_path(const struct path *path, struct path *root,
bool slash = false;
int error = 0;
spin_lock(&vfsmount_lock);
br_read_lock(vfsmount_lock);
while (dentry != root->dentry || vfsmnt != root->mnt) {
struct dentry * parent;
@ -1964,7 +1964,7 @@ out:
if (!error && !slash)
error = prepend(buffer, buflen, "/", 1);
spin_unlock(&vfsmount_lock);
br_read_unlock(vfsmount_lock);
return error;
global_root:
@ -2302,11 +2302,12 @@ int path_is_under(struct path *path1, struct path *path2)
struct vfsmount *mnt = path1->mnt;
struct dentry *dentry = path1->dentry;
int res;
spin_lock(&vfsmount_lock);
br_read_lock(vfsmount_lock);
if (mnt != path2->mnt) {
for (;;) {
if (mnt->mnt_parent == mnt) {
spin_unlock(&vfsmount_lock);
br_read_unlock(vfsmount_lock);
return 0;
}
if (mnt->mnt_parent == path2->mnt)
@ -2316,7 +2317,7 @@ int path_is_under(struct path *path1, struct path *path2)
dentry = mnt->mnt_mountpoint;
}
res = is_subdir(dentry, path2->dentry);
spin_unlock(&vfsmount_lock);
br_read_unlock(vfsmount_lock);
return res;
}
EXPORT_SYMBOL(path_is_under);