On Tue, Sep 10, 2013 at 11:43 AM, Al Viro <v...@zeniv.linux.org.uk> wrote: > > !LOOKUP_ROOT: we set nd->root the first time we need / (in the very > beginning if it's an absolute pathname, on the first absolute symlink > otherwise). In non-RCU mode we hold a reference to it; in RCU mode > we do not. As the result, leaving RCU mode should either grab > a reference to the damn thing (if we intend to go on) or zero it out.
Yeah, that was what I was thinking. But in particular, I was wondering if we could simplify unlazy_walk() to _not_ take that root reference at all, and just always zero it out even if we succeed. IOW, doing the attached patch (_instead_ of the one I sent out). Or is there something in path lookup retrying that might get uphappy if we go back to a NULL root.mnt, and doesn't check it? Because this patch actually simplifies that nasty/complex unlazy_walk() a lot, and makes it much more understnadable, I think. It always looked really odd to me how it used to do "if LOOKUP_ROOT is _not_ set, let's take a reference count to the root"). With this patch in place, I really like how straightforward unlazy_walk() is. That used to be just about _the_ most subtle part of the whole rcu-to-refcount thing (as demonstrated by the number of bugs it has exposed in the patches), and now it looks almost trivial. Mace - ignore this newer patch for now, the one I want you to test is the minimal one you already have, not this cleanup one. But since you clearly saw the problem I never did, if you get bored _after_ testing the first patch, feel free to give this one a whirl too. But notice that this patch is a replacement for - not in addition to - the first one. Linus
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