Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sun, Sep 01, 2013 at 01:59:22PM -0700, Linus Torvalds wrote:

> [ Side note: Al, that lg_local_lock really is annoying: it's
> br_read_lock(mntput_no_expire), with two thirds of the calls coming
> from mntput_no_expire, and the rest from path_init -> lock_rcu_walk.

How much of that is due to br_write_lock() taken in mntput_no_expire()
for no good reason?  IOW, could you try shmem.c patch I've sent yesterday
and see how much effect does it have?[1]  Basically, we get it grabbed
exclusive on each final fput() of a struct file created by shmem_file_setup(),
which is _not_ a rare event.  And the only reason for that is not having
shm_mnt marked long-living, even though its refcount never hits 0...
 
>   I really really wonder if we could get rid of the
> br_read_lock(&vfsmount_lock) for rcu_walk_init(), and use just the RCU
> read accesses for the mount-namespaces too. What is that lock really
> protecting against during lookup anyway? ]

A lot of things, I'm afraid.  It's not as simple as just the access
to vfsmount hash... ;-/  I'll need to do some digging to put together
a full analysis, but there had been quite a few subtle issues where
it played...

[1] sits in the local queue, will push tonight:

commit e7db6c4c1d01032f53262f03b5f38899f9db8add
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sat Aug 31 12:57:10 2013 -0400

shm_mnt is as longterm as it gets, TYVM...
    
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/mm/shmem.c b/mm/shmem.c
index e43dc55..5261498 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2615,13 +2615,15 @@ int shmem_fill_super(struct super_block *sb, void *data, int silent)
 	 * tmpfs instance, limiting inodes to one per page of lowmem;
 	 * but the internal instance is left unlimited.
 	 */
-	if (!(sb->s_flags & MS_NOUSER)) {
+	if (!(sb->s_flags & MS_KERNMOUNT)) {
 		sbinfo->max_blocks = shmem_default_max_blocks();
 		sbinfo->max_inodes = shmem_default_max_inodes();
 		if (shmem_parse_options(data, sbinfo, false)) {
 			err = -EINVAL;
 			goto failed;
 		}
+	} else {
+		sb->s_flags |= MS_NOUSER;
 	}
 	sb->s_export_op = &shmem_export_ops;
 	sb->s_flags |= MS_NOSEC;
@@ -2831,8 +2833,7 @@ int __init shmem_init(void)
 		goto out2;
 	}
 
-	shm_mnt = vfs_kern_mount(&shmem_fs_type, MS_NOUSER,
-				 shmem_fs_type.name, NULL);
+	shm_mnt = kern_mount(&shmem_fs_type);
 	if (IS_ERR(shm_mnt)) {
 		error = PTR_ERR(shm_mnt);
 		printk(KERN_ERR "Could not kern_mount tmpfs\n");