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    SubjectRe: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)
    On 05/03, Peter Zijlstra wrote:
    >
    > On Thu, May 02, 2019 at 12:09:32PM +0200, Oleg Nesterov wrote:
    >
    > > > +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.

    Of course. I meant that the next writer can enter block_readers() if
    up_non_owner() drops ->rw_sem, but it will block in wait_event(!readers_block).

    (And if we change this code to use wait_event(xchg(readers_block) == 0) we
    can remove rw_sem altogether).

    The main problem is that this is sub-optimal. We can have a lot of readers
    sleeping in __down_read() when percpu_down_write() succeeds, then after
    percpu_down_write_non_owner() does up_write() they all will be woken just
    to hang in readers_block(). Plus the new readers will need to pass the
    lock-check-unlock-schedule path.

    Peter, just in case... I see another patch from you but I need to run away
    till Monday.

    Oleg.

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