On 21.02.2015 02:50, Al Viro wrote:
On Fri, Feb 20, 2015 at 03:07:31PM -0800, Andrew Morton wrote:
- It no longer "acquires a reference". All it does is to acquire an rwsem.
- What the heck is a "passive reference" anyway? It appears to be
the situation where we increment s_count without incrementing s_active.
Reference to struct super_block that guarantees only that its memory won't
be freed until we drop it.
After your patch, this superblock state no longer exists(?),
Yes, it does. The _only_ reason why that patch isn't outright bogus is that
we do only down_read_trylock() on ->s_umount - try to pull off the same thing
with down_read() and you'll get a nasty race.
I don't get this. What the problem with down_read(sb->s_umount)?
For grab_super_passive()/trylock_super() caller guarantees memory
wouldn't be freed and we check tsb activeness after grabbing shared
lock. And while we hold that lock it'll stay active.
It have to use down_read_trylock() just because it works in in atomic
context when writeback calls it. No?
Check for activeness actually is a quite confusing.
It seems checking for MS_BORN and MS_ACTIVE should be enough:
bool trylock_super(struct super_block *sb)
{
if (down_read_trylock(&sb->s_umount)) {
- if (!hlist_unhashed(&sb->s_instances) &&
- sb->s_root && (sb->s_flags & MS_BORN))
+ if ((sb->s_flags & MS_BORN) && (sb->s_flags & MS_ACTIVE))
return true;
up_read(&sb->s_umount);
}
Take a look at e.g.
get_super(). Or user_get_super(). Or iterate_supers()/iterate_supers_type(),
where we don't return such references, but pass them to a callback instead.
In all those cases we end up with passive reference taken, ->s_umount
taken shared (_NOT_ with trylock) and fs checked for being still alive.
Then it's guaranteed to stay alive until we do drop_super().
I agree that the name blows, BTW - something like try_get_super() might have
been more descriptive, but with this change it actually becomes a bad name
as well, since after it we need a different way to release the obtained ref;
not the same as after get_super(). Your variant might be OK, but I'd
probably make it trylock_super(), to match the verb-object order of the
rest of identifiers in that area...
so
perhaps the entire "passive reference" concept and any references to
it can be expunged from the kernel.
Nope.
--
Konstantin
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