Re: [PATCH] fs: avoid locking sb_lock in grab_super_passive()

From: Al Viro
Date: Fri Feb 20 2015 - 18:50:25 EST

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. 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.

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