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Date: Tue, 31 Jan 2017 15:26:37 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Waiman Long <longman@...hat.com> Cc: Jens Axboe <axboe@...com>, "J. R. Okajima" <hooanon05g@...il.com>, linux-kernel@...r.kernel.org, darrick.wong@...cle.com, david@...morbit.com, dave@...olabs.net Subject: Re: Q: lockdep_assert_held_read() after downgrade_write() On Tue, Jan 31, 2017 at 09:23:08AM -0500, Waiman Long wrote: > On 01/31/2017 06:25 AM, Peter Zijlstra wrote: > > @@ -40,8 +40,10 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) > > * do a write to the rwsem cacheline when it is really necessary > > * to minimize cacheline contention. > > */ > > - if (sem->owner != RWSEM_READER_OWNED) > > + if (sem->owner != RWSEM_READER_OWNED) { > > + WARN_ON_ONCE(sem->owner != current); > > WRITE_ONCE(sem->owner, RWSEM_READER_OWNED); > > + } > > } > > > > static inline bool rwsem_owner_is_writer(struct task_struct *owner) > > I don't think you can do a WARN_ON_ONCE() check for sem->owner != > current here. If the rwsem starts from an unlock state, sem->owner will > be NULL and an incorrect warning message will be printed. Argh, I only looked at the downgrade_write() user and forgot to look if it was used elsewhere.
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