Messages in this thread | | | From | Nix <> | Subject | Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug) | Date | Tue, 23 Oct 2012 18:05:18 +0100 |
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On 23 Oct 2012, Trond Myklebust spake thusly: > On Tue, 2012-10-23 at 12:46 -0400, J. Bruce Fields wrote: >> Looks like there's some confusion about whether nsm_client_get() returns >> NULL or an error? > > nsm_client_get() looks extremely racy in the case where ln->nsm_users == > 0. Since we never recheck the value of ln->nsm_users after taking > nsm_create_mutex, what is stopping 2 different threads from both setting > ln->nsm_clnt and re-initialising ln->nsm_users?
Yep. At the worst possible time:
spin_lock(&ln->nsm_clnt_lock); if (ln->nsm_users) { if (--ln->nsm_users) ln->nsm_clnt = NULL; (1) shutdown = !ln->nsm_users; } spin_unlock(&ln->nsm_clnt_lock);
If a thread reinitializes nsm_users at point (1), after the assignment, we could well end up with ln->nsm_clnt NULL and shutdown false. A bit later, nsm_mon_unmon gets called with a NULL clnt, and boom.
This seems particularly likely if there is only one nsm_user (which is true in my case, since I have only one active network namespace).
-- NULL && (void)
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