Re: [PATCH] fix race in mark_mounts_for_expiry()

[Miklos Szeredi]

> But _only after_ it's has gone to zero.  When in fact there are no
> more references to it, so it shouldn't matter.
> 
> The fact that it does matter and that mark_mounts_for_expiry()
> derefences mnt->mnt_namespace without actually having a proper
> reference to the namespace is the real culprit here.
> 
> This is the third bug found by Jamie Lokier, Ram and me in the
> mnt_namespace change.  So if we are looking at proper solutions I
> think that is what we should be examining.

E.g. having a separate task count, which is incremented/decremented
only by clone/exit.  If the task count goes to zero, umount_tree is
called on root, but namespace is not freed.

And each mnt_namespace holds a proper reference to the namespace, so
it's safe to dereference it anytime.  When truly no more references
remain, the namespace can go away.

Hmm?

Miklos
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