Messages in this thread | | | Date | Fri, 3 May 2019 16:16:33 +0200 | From | Peter Zijlstra <> | Subject | Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16) |
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On Thu, May 02, 2019 at 12:09:32PM +0200, Oleg Nesterov wrote: > On 05/01, Peter Zijlstra wrote: > > > > Anyway; I cobbled together the below. Oleg, could you have a look, I'm > > sure I messed it up. > > Oh, I will need to read this carefully. but at first glance I do not see > any hole... > > > +static void readers_block(struct percpu_rw_semaphore *sem) > > +{ > > + wait_event_cmd(sem->writer, !sem->readers_block, > > + __up_read(&sem->rw_sem), __down_read(&sem->rw_sem)); > > +} > > + > > +static void block_readers(struct percpu_rw_semaphore *sem) > > +{ > > + wait_event_exclusive_cmd(sem->writer, !sem->readers_block, > > + __up_write(&sem->rw_sem), > > + __down_write(&sem->rw_sem)); > > + /* > > + * Notify new readers to block; up until now, and thus throughout the > > + * longish rcu_sync_enter() above, new readers could still come in. > > + */ > > + WRITE_ONCE(sem->readers_block, 1); > > +} > > So iiuc, despite it name block_readers() also serializes the writers, ->rw_sem > can be dropped by down_write_non_owner() so the new writer can take this lock.
I don't think block_readers() is sufficient to serialize writers; suppose two concurrent callers when !->readers_block. Without ->rwsem that case would not serialize.
> But this all is cosmetic, it seems that we can remove ->rw_sem altogether > but I am not sure...
Only if we introduce something like ->wait_lock to serialize things.
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