On Fri, 13 Feb 2015, Josh Poimboeuf wrote: > On Fri, Feb 13, 2015 at 05:17:10PM +0100, Miroslav Benes wrote: > > On Fri, 13 Feb 2015, Josh Poimboeuf wrote: > > > Hm, even with Jiri Slaby's suggested fix to add the completion to the > > > unregister path, I still get a lockdep warning. This looks more > > > insidious, > > > related to the locking order of a kernfs lock and the klp lock. I'll > > > need to > > > look at this some more... > > > > Yes, I was afraid of this. Lockdep warning is a separate bug. It is caused > > by taking klp_mutex in enabled_store. During rmmod klp_unregister_patch > > takes klp_mutex and destroys the sysfs structure. If somebody writes to > > enabled just after unregister takes the mutex and before the sysfs > > removal, he would cause the deadlock, because enabled_store takes the > > "sysfs lock" and then klp_mutex. That is exactly what the lockdep tells us > > below. > > > > We can look for inspiration elsewhere. Grep for s_active through git log > > of the mainline offers several commits which dealt exactly with this. Will > > browse through that... > > Thanks Miroslav, please let me know what you find. It wouldn't surprise > me if this were a very common problem. > > One option would be to move the enabled_store() work out to a workqueue > or something.
Yes, that is one possibility. It is not the only one. 1. we could replace mutex_lock in enabled_store with mutex_trylock. If the lock was not acquired we would return -EBUSY. Or could we 'return restart_syscall' (maybe after some tiny msleep)? 2. we could reorganize klp_unregister_patch somehow and move sysfs removal out of mutex protection. Miroslav > > > > > > To recreate: > > > > > > insmod livepatch-sample.ko > > > > > > # wait for patching to complete > > > > > > ~/a.out & <-- simple program which opens the "enabled" file in the > > > background > > > > I didn't even need such a program. Lockdep warned me with sole insmod, > > echo and rmmod. It is magically clever. > > Ah, even easier... lockdep is awesome. > > -- > Josh > -- Miroslav Benes SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
