Hi Josh, on 2015/12/1 9:11, Josh Poimboeuf wrote: > On Mon, Nov 30, 2015 at 11:54:37AM +0800, Li Bin wrote: >> There is a potential race as following: >> >> CPU0 | CPU1 >> -----------------------------|----------------------------------- >> enabled_store() | klp_unregister_patch() >> | |-mutex_lock(&klp_mutex); >> |-mutex_lock(&klp_mutex); | |-klp_free_patch(); >> | |-mutex_unlock(&klp_mutex); >> |-[process the patch's state]| >> |-mutex_unlock(&klp_mutex) | >> >> Fix this race condition by adding klp_is_patch_registered() check in >> enabled_store() after get the lock klp_mutex. > I'm thinking this race isn't possible, and that instead it would > deadlock. Good point. I did not consider the kernfs lock. > When I try to recreate something similar by putting a delay in > enabled_store(), klp_free_patch() just sleeps on its call to > kobject_put() until enabled_store() returns. The unregister stack looks > like: > > [<ffffffff812e966b>] __kernfs_remove+0x1fb/0x380 > [<ffffffff812ea273>] kernfs_remove+0x23/0x40 > [<ffffffff812ec601>] sysfs_remove_dir+0x51/0x80 > [<ffffffff81407fb8>] kobject_del+0x18/0x50 > [<ffffffff8140804a>] kobject_release+0x5a/0x190 > [<ffffffff81407f27>] kobject_put+0x27/0x50 > [<ffffffff81128d41>] klp_unregister_patch+0x71/0x80 > > It seems to be waiting on a lock which is held by the kernfs code which > called enabled_store(). So when enabled_store() tries to get the > klp_mutex, it deadlocks. > > Miroslav and I previously discussed a few options to fix this: > > https://lkml.kernel.org/r/alpine.LNX.2.00.1502171728520.29490@xxxxxxxxxxxxx Both the options seem good to me. I look forward to the patch. :) Thanks, Li Bin -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
