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 Unless I am totally confused read-after-read is no longer valid, 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
