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In [1], Will raised a potential issue that the cfg80211 code, which does (from a locking perspective) rtnl_lock() wiphy_lock() rtnl_unlock() might be suspectible to ABBA deadlocks, because rtnl_unlock() calls netdev_run_todo(), which might end up calling rtnl_lock() again, which could then deadlock (see the comment in the code added here for the scenario). Some back and forth and thinking ensued, but clearly this can't happen if the net_todo_list is empty at the rtnl_unlock() here. Clearly, the code here cannot actually put an entry on it, and all other users of rtnl_unlock() will empty it since that will always go through netdev_run_todo(), emptying the list. So the only other way to get there would be to add to the list and then unlock the RTNL without going through rtnl_unlock(), which is only possible through __rtnl_unlock(). However, this isn't exported and not used in many places, and none of them seem to be able to unregister before using it. Therefore, add a WARN_ON() in the code to ensure this invariant won't be broken, so that the cfg80211 (or any similar) code stays safe. [1] https://lore.kernel.org/r/Yjzpo3TfZxtKPMAG@google.com Signed-off-by: Johannes Berg <johannes.berg@intel.com> Link: https://lore.kernel.org/r/20220404113847.0ee02e4a70da.Ic73d206e217db20fd22dcec14fe5442ca732804b@changeid Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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