• Jason A. Donenfeld's avatar
    padata: avoid race in reordering · de5540d0
    Jason A. Donenfeld authored
    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
    [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:
    list_add_tail(&padata->list, &squeue->serial.list);
    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);
           goto 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: default avatarJason A. Donenfeld <Jason@zx2c4.com>
    Acked-by: default avatarSteffen Klassert <steffen.klassert@secunet.com>
    Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
padata.c 25.4 KB