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. 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 -- Josh -- 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
