Al Viro <v...@zeniv.linux.org.uk> writes:
> On Tue, Apr 19, 2016 at 10:43:03PM -0500, Eric W. Biederman wrote:
>> >> + if (!d_can_lookup(parent))
>> >> + return -ENOENT;
>> >
>> > And how, pray tell, would a parent of anything fail to be a directory?
>>
>> It is to make that function be visually distinct from path_parentat
>> which does something rather different.
>
> Huh? I'm asking how can that condition ever turn out to be true. Unless
> you really advocate something like
> if (2 * 17 != 34)
> return -234567; // to make it visually distinct from foobar(),
> // which doesn't have such a test
> your reply doesn't seem to make any sense...
Oh apologies I thought you were asking about the naming of the function,
path_parent_directory. Yes. The d_can_lookup does appear to be redundant.
It definitely looks like bedtime for me.
>> >> + this.name = "pts";
>> >> + this.len = 3;
>> >> + this.hash = full_name_hash(this.name, this.len);
>> >> + if (parent->d_flags & DCACHE_OP_HASH) {
>> >> + int err = parent->d_op->d_hash(parent, &this);
>> >> + if (err < 0)
>> >> + return err;
>> >> + }
>> >> + inode_lock(parent->d_inode);
>> >
>> > What the hell for? What does that lock on parent change for the
>> > dcache lookup you are doing here?
>>
>> Good point. That is overkill. As we know the dentry is a mount point and
>> must be in the dcache, the customary lock for performing a lookup from
>> the disk is not necessary.
>
> Er... To avoid reader confusion:
> a) d_lookup() does *not* do a filesystem lookup
> b) it does not need inode_lock()
> c) it (and not a "lookup from the disk") is what's actually being
> called in the code in question.
And since I was stripping down the ordinary filesystem lookup path to
just the pieces needed I apparently wound up with a few extras.
Do you think it would be possible to guarantee an rcu lookup for the
operations in path_pts? I think needing to perform a follow_mount makes
that impossible to guarantee.
All the caller wants is to find the superblock of the mounted filesystem
and increment sb->s_active.
Eric
