[+Boqun] On Wed, May 23, 2018 at 08:25:06AM -0700, Linus Torvalds wrote: > On Wed, May 23, 2018 at 6:05 AM Will Deacon <will.dea...@arm.com> wrote: > > > Please use a newer kernel. We've addressed this in mainline by moving > > arm64 over to the qrwlock implementation which (after some other changes) > > guarantees forward progress for well-behaved readers and writers. > > Oh, I didn't even realize that this wasn't x86, and that there was still > the very unfair rwlock issue on 4.14 on arm. > > Yeah, the queuing rwlocks shouldn't show the really pathological problems > we used to have long ago.
Yup, although they do reveal new issues that Boqun has been running into recently with his lockdep improvements. The general pattern is if you have: P0: P1: P2: spin_lock(&slock) read_lock(&rwlock) write_lock(&rwlock) read_lock(&rwlock) spin_lock(&slock) then the CPUs can be queued on the rwlock such that P1 has the lock, then P2 is queued and then P0. If P0 has taken the spinlock, we have a deadlock which wouldn't arise with the non-queued version. In other words, qrwlock requires consistent locking order wrt spinlocks. Will
