On Thu, Mar 23, 2017 at 12:24:43PM +0100, Jason A. Donenfeld wrote: > Under extremely heavy uses of padata, crashes occur, and with list > debugging turned on, this happens instead: > > [87487.298728] WARNING: CPU: 1 PID: 882 at lib/list_debug.c:33 > __list_add+0xae/0x130 > [87487.301868] list_add corruption. prev->next should be next > (ffffb17abfc043d0), but was ffff8dba70872c80. (prev=ffff8dba70872b00). > [87487.339011] [<ffffffff9a53d075>] dump_stack+0x68/0xa3 > [87487.342198] [<ffffffff99e119a1>] ? console_unlock+0x281/0x6d0 > [87487.345364] [<ffffffff99d6b91f>] __warn+0xff/0x140 > [87487.348513] [<ffffffff99d6b9aa>] warn_slowpath_fmt+0x4a/0x50 > [87487.351659] [<ffffffff9a58b5de>] __list_add+0xae/0x130 > [87487.354772] [<ffffffff9add5094>] ? _raw_spin_lock+0x64/0x70 > [87487.357915] [<ffffffff99eefd66>] padata_reorder+0x1e6/0x420 > [87487.361084] [<ffffffff99ef0055>] padata_do_serial+0xa5/0x120 > > padata_reorder calls list_add_tail with the list to which its adding > locked, which seems correct: > > spin_lock(&squeue->serial.lock); > list_add_tail(&padata->list, &squeue->serial.list); > spin_unlock(&squeue->serial.lock); > > This therefore leaves only place where such inconsistency could occur: > if padata->list is added at the same time on two different threads. > This pdata pointer comes from the function call to > padata_get_next(pd), which has in it the following block: > > next_queue = per_cpu_ptr(pd->pqueue, cpu); > padata = NULL; > reorder = &next_queue->reorder; > if (!list_empty(&reorder->list)) { > padata = list_entry(reorder->list.next, > struct padata_priv, list); > spin_lock(&reorder->lock); > list_del_init(&padata->list); > atomic_dec(&pd->reorder_objects); > spin_unlock(&reorder->lock); > > pd->processed++; > > goto out; > } > out: > return padata; > > I strongly suspect that the problem here is that two threads can race > on reorder list. Even though the deletion is locked, call to > list_entry is not locked, which means it's feasible that two threads > pick up the same padata object and subsequently call list_add_tail on > them at the same time. The fix is thus be hoist that lock outside of > that block. > > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Acked-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>