On Thu, Sep 27, 2012 at 9:16 AM, Myklebust, Trond
<Trond.Myklebust@xxxxxxxxxx> wrote:
>
> I cannot see how that BUG_ON can be triggered in the current code, given
> that the only place where idmap->idmap_key_cons is set to a non-NULL
> value is covered by a mutex, and that it is always cleared before we
> release said mutex.
Quite frankly, the "I cannot see" thing is *never* an excuse for a BUG_ON().
We don't do kernel-killing asserts in Linux. Never.
The only excuse for a BUG_ON() is "I cannot possibly continue, I don't
even have an error path I can take".
If it's a fundamentally impossible situation, the BUG_ON() should
never have been there in the first place!
And if it's a "I don't see how it could happen", then it should have
been something like
if (WARN_ON_ONCE(condition))
goto cleanup;
rather than a BUG_ON().
We have too many f*cking BUG_ON's in the kernel, and the fact that one
triggers and it has taken a month and a half without it even being
resolved is a problem.
Get rid of the thing, already, dammit. If you cannot figure out how it
can happen, then the *last* thing you want to do is then kill the
machine so that it's impossible to debug it sanely.
Besides, as far as I can tell, idmap_key_cons locking is suspect
anyway. Stuff like this:
cons = ACCESS_ONCE(idmap->idmap_key_cons);
idmap->idmap_key_cons = NULL;
is an almost certain example of "the code is racy, and we did it
wrong". The above is basically *never* correct.
If the access is properly locked, then the ACCESS_ONCE() is a bug.
And if the access *isn't* properly locked, then setting things to NULL
afterwards is in no way safe.
IOW, either way, it's broken. And there's at least two of those
clearly buggy code-sequences involving that field.
So get rid of the BUG_ON() (possibly replacing it with the
WARN_ON_ONCE), and please look at those ACCESS_ONCE() sequences and
fix them. Either they happen under a lock, or they don't. None of this
crazy racy crap, please.
Linus
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