On Thu, May 09, 2019 at 05:56:46PM +0200, Oleg Nesterov wrote: > Impossible ;) I bet lockdep should report the deadlock as soon as find_victims() > calls find_lock_task_mm() when you already have a locked victim. I hope you're not a betting man ;) With the following configured: CONFIG_DEBUG_RT_MUTEXES=y CONFIG_DEBUG_SPINLOCK=y # CONFIG_DEBUG_SPINLOCK_BITE_ON_BUG is not set CONFIG_DEBUG_SPINLOCK_PANIC_ON_BUG=y CONFIG_DEBUG_MUTEXES=y CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y CONFIG_LOCKDEP=y CONFIG_LOCK_STAT=y CONFIG_DEBUG_LOCKDEP=y CONFIG_DEBUG_ATOMIC_SLEEP=y # CONFIG_DEBUG_LOCKING_API_SELFTESTS is not set # CONFIG_LOCK_TORTURE_TEST is not set And a printk added in vtsk_is_duplicate() to print when a duplicate is detected, and my phone's memory cut in half to make simple_lmk do something, this is what I observed: taimen:/ # dmesg | grep lockdep [ 0.000000] \x09RCU lockdep checking is enabled. taimen:/ # dmesg | grep simple_lmk [ 23.211091] simple_lmk: Killing android.carrier with adj 906 to free 37420 kiB [ 23.211160] simple_lmk: Killing oadcastreceiver with adj 906 to free 36784 kiB [ 23.248457] simple_lmk: Killing .apps.translate with adj 904 to free 45884 kiB [ 23.248720] simple_lmk: Killing ndroid.settings with adj 904 to free 42868 kiB [ 23.313417] simple_lmk: DUPLICATE VTSK! [ 23.313440] simple_lmk: Killing ndroid.keychain with adj 906 to free 33680 kiB [ 23.313513] simple_lmk: Killing com.whatsapp with adj 904 to free 51436 kiB [ 34.646695] simple_lmk: DUPLICATE VTSK! [ 34.646717] simple_lmk: Killing ndroid.apps.gcs with adj 906 to free 37956 kiB [ 34.646792] simple_lmk: Killing droid.apps.maps with adj 904 to free 63600 kiB taimen:/ # dmesg | grep lockdep [ 0.000000] \x09RCU lockdep checking is enabled. taimen:/ # > As for https://github.com/kerneltoast/android_kernel_google_wahoo/commit/afc8c9bf2dbde95941253c168d1adb64cfa2e3ad > Well, > > mmdrop(mm); > simple_lmk_mm_freed(mm); > > looks racy because mmdrop(mm) can free this mm_struct. Yes, simple_lmk_mm_freed() > does not dereference this pointer, but the same memory can be re-allocated as > another ->mm for the new task which can be found by find_victims(), and _in theory_ > this all can happen in between, so the "victims[i].mm == mm" can be false positive. > > And this also means that simple_lmk_mm_freed() should clear victims[i].mm when > it detects "victims[i].mm == mm", otherwise we have the same theoretical race, > victims_to_kill is only cleared when the last victim goes away. Fair point. Putting simple_lmk_mm_freed(mm) right before mmdrop(mm), and sprinkling in a cmpxchg in simple_lmk_mm_freed() should fix that up. > Another nit... you can drop tasklist_lock right after the 1st "find_victims" loop. True! > And it seems that you do not really need to walk the "victims" array twice after that, > you can do everything in a single loop, but this is cosmetic. Won't this result in potentially holding the task lock way longer than necessary for multiple tasks that aren't going to be killed? Thanks, Sultan
