seq_lock and lockdep_is_held() assertions

[Jann Horn]

adding some locking folks to the thread...

On Fri, Feb 21, 2020 at 6:06 PM David Howells <dhowells@xxxxxxxxxx> wrote:
> Jann Horn <jannh@xxxxxxxxxx> wrote:
> > On Fri, Feb 21, 2020 at 1:24 PM David Howells <dhowells@xxxxxxxxxx> wrote:
> > > What's the best way to write a lockdep assertion?
> > >
> > >         BUG_ON(!lockdep_is_held(lock));
> >
> > lockdep_assert_held(lock) is the normal way, I think - that will
> > WARN() if lockdep is enabled and the lock is not held.
>
> Okay.  But what's the best way with a seqlock_t?  It has two dep maps in it.
> Do I just ignore the one attached to the spinlock?

Uuuh... very good question. Looking at how the seqlock_t helpers use
the dep map of the seqlock, I don't think lockdep asserts work for
asserting that you're in the read side of a seqlock?

read_seqbegin_or_lock() -> read_seqbegin() -> read_seqcount_begin() ->
seqcount_lockdep_reader_access() does seqcount_acquire_read() (which
maps to lock_acquire_shared_recursive()), but immediately following
that calls seqcount_release() (which maps to lock_release())?

So I think lockdep won't consider you to be holding any locks after
read_seqbegin_or_lock() if the lock wasn't taken?