Messages in this thread | | | Date | Mon, 6 May 2019 18:50:09 +0200 | From | Oleg Nesterov <> | Subject | Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16) |
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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.
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