On 05/03, Peter Zijlstra wrote:
>
> -static void lockdep_sb_freeze_release(struct super_block *sb)
> -{
> - int level;
> -
> - for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
> - percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> -}
> -
> -/*
> - * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
> - */
> -static void lockdep_sb_freeze_acquire(struct super_block *sb)
> -{
> - int level;
> -
> - for (level = 0; level < SB_FREEZE_LEVELS; ++level)
> - percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> + percpu_down_write_non_owner(sb->s_writers.rw_sem + level-1);
> }
I'd suggest to not change fs/super.c, keep these helpers, and even not introduce
xxx_write_non_owner().
freeze_super() takes other locks, it calls sync_filesystem(), freeze_fs(), lockdep
should know that this task holds SB_FREEZE_XXX locks for writing.
> @@ -80,14 +83,8 @@ int __percpu_down_read(struct percpu_rw_
> * and reschedule on the preempt_enable() in percpu_down_read().
> */
> preempt_enable_no_resched();
> -
> - /*
> - * Avoid lockdep for the down/up_read() we already have them.
> - */
> - __down_read(&sem->rw_sem);
> + wait_event(sem->waiters, !atomic_read(&sem->block));
> this_cpu_inc(*sem->read_count);
Argh, this looks racy :/
Suppose that sem->block == 0 when wait_event() is called, iow the writer released
the lock.
Now suppose that this __percpu_down_read() races with another percpu_down_write().
The new writer can set sem->block == 1 and call readers_active_check() in between,
after wait_event() and before this_cpu_inc(*sem->read_count).
Oleg.
