Re: qrwlock && read-after-read

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 04, 2015 at 03:00:53PM +0200, Oleg Nesterov wrote:
> I am working on the (off-topic) bug report which motivated me to
> look at locking/qrwlock.c and it seems to me there is a problem
> with the queued rwlocks.
> 
> Unless I am totally confused read-after-read is no longer valid,
> write_lock() stops the new readers. And lockdep doesn't know this,
> read_lock()->rwlock_acquire_read() doesn't match the reality. The
> code doing
> 
> 	read_lock(X);
> 	read_lock(X);
> 
> can deadlock if another CPU does write_lock(X) in between. This
> was fine before rwlock_t was changed to use qrwlock.
> 
> A nested read_lock() in interrupt should be fine though, and this
> is because queue_read_lock_slowpath() "ignores" _QW_WAITING if
> in_interrupt().
> 
> This means that rwlock_t has the really strange semantics imho,
> and again, it is not lockdep-friendly.
> 
> What do you think we can/should do? Or did I misread this code?

Fix lockdep, although that's non trivial from what I remember.

These (new) semantics were very much on purpose and suggested by Linus
IIRC.