On 05/05/2011 08:09 PM, Jan Kara wrote: > Hello, > > On Thu 05-05-11 17:57:16, Tao Ma wrote: >> From: Tao Ma <boyu.mt@xxxxxxxxxx> >> >> In do_get_write_access, we check journal_head->b_jlist and if it >> is BJ_Shadow, we will sleep until we remove it from t_shadow_list >> in jbd2_journal_commit_transaction, but it isn't protected by any >> lock. So if we uses some cached b_jlist and before schedule, >> jbd2_journal_commit_transaction has already waken up all >> the waiting thread. As a result, this thread will never be waken up. > I had a look at the code and I think it's more complicated than this. > The code is: > prepare_to_wait(wqh, &wait.wait, TASK_UNINTERRUPTIBLE); > if (jh->b_jlist != BJ_Shadow) > break; > schedule(); > > You're right that jh->b_jlist != BJ_Shadow test is done without any lock. > But prepare_to_wait() does set_current_state() which implies a memory > barrier. The comment there says: > /* > * set_current_state() includes a barrier so that the write of current->state > * is correctly serialised wrt the caller's subsequent test of whether to > * actually sleep: > * > * set_current_state(TASK_UNINTERRUPTIBLE); > * if (do_i_need_to_sleep()) > * schedule(); > * > * If the caller does not need such serialisation then use __set_current_state() > */ > So we are guaranteed that either we see that jh->b_jlist != BJ_Shadow or > the waking process sees us in the wait queue and removes us. > > Well, not quite. The waking code is: > journal_file_buffer(jh, commit_transaction, BJ_Forget); > /* Wake up any transactions which were waiting for this > IO to complete */ > wake_up_bit(&bh->b_state, BH_Unshadow); > And that's where the problem actually is. Even the comment before > wake_up_bit() warns that: > * In order for this to function properly, as it uses waitqueue_active() > * internally, some kind of memory barrier must be done prior to calling > * this. Typically, this will be smp_mb__after_clear_bit(), but in some > * cases where bitflags are manipulated non-atomically under a lock, one > * may need to use a less regular barrier, such fs/inode.c's smp_mb(), > * because spin_unlock() does not guarantee a memory barrier. > I'll send proper fix in a moment. oh, great thanks for the fix and the detailed explanation about the memory barrier. Regards, Tao -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
