On Mon, Jun 13 2022 at 16:07, Sebastian Andrzej Siewior wrote: Polite ping .... > PREEMPT_RT has two issues with the dcache code: > > 1) i_dir_seq is a special cased sequence counter which uses the lowest > bit as writer lock. This requires that preemption is disabled. > > On !RT kernels preemption is implictly disabled by spin_lock(), but > that's not the case on RT kernels. > > Replacing i_dir_seq on RT with a seqlock_t comes with its own > problems due to arbitrary lock nesting. Using a seqcount with an > associated lock is not possible either because the locks held by the > callers are not necessarily related. > > Explicitly disabling preemption on RT kernels across the i_dir_seq > write held critical section is the obvious and simplest solution. The > critical section is small, so latency is not really a concern. > > 2) The wake up of dentry::d_wait waiters is in a preemption disabled > section, which violates the RT constraints as wake_up_all() has > to acquire the wait queue head lock which is a 'sleeping' spinlock > on RT. > > There are two reasons for the non-preemtible section: > > A) The wake up happens under the hash list bit lock > > B) The wake up happens inside the i_dir_seq write side > critical section > > #A is solvable by moving it outside of the hash list bit lock held > section. > > Making #B preemptible on RT is hard or even impossible due to lock > nesting constraints. > > A possible solution would be to replace the waitqueue by a simple > waitqueue which can be woken up inside atomic sections on RT. > > But aside of Linus not being a fan of simple waitqueues, there is > another observation vs. this wake up. It's likely for the woken up > waiter to immediately contend on dentry::lock. > > It turns out that there is no reason to do the wake up within the > i_dir_seq write held region. The only requirement is to do the wake > up within the dentry::lock held region. Further details in the > individual patches. > > That allows to move the wake up out of the non-preemptible section > on RT, which also reduces the dentry::lock held time after wake up. > > Thanks, > > Sebastian
