On Fri, Mar 20, 2020 at 08:51:44PM +0100, Thomas Gleixner wrote: > "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes: > > > > - The soft interrupt related suffix (_bh()) still disables softirq > > handlers. However, unlike non-PREEMPT_RT kernels (which disable > > preemption to get this effect), PREEMPT_RT kernels use a per-CPU > > lock to exclude softirq handlers. > > I've made that: > > - The soft interrupt related suffix (_bh()) still disables softirq > handlers. > > Non-PREEMPT_RT kernels disable preemption to get this effect. > > PREEMPT_RT kernels use a per-CPU lock for serialization. The lock > disables softirq handlers and prevents reentrancy by a preempting > task. That works! At the end, I would instead say "prevents reentrancy due to task preemption", but what you have works. > On non-RT this is implicit through preemption disable, but it's non > obvious for RT as preemption stays enabled. > > > PREEMPT_RT kernels preserve all other spinlock_t semantics: > > > > - Tasks holding a spinlock_t do not migrate. Non-PREEMPT_RT kernels > > avoid migration by disabling preemption. PREEMPT_RT kernels instead > > disable migration, which ensures that pointers to per-CPU variables > > remain valid even if the task is preempted. > > > > - Task state is preserved across spinlock acquisition, ensuring that the > > task-state rules apply to all kernel configurations. In non-PREEMPT_RT > > kernels leave task state untouched. However, PREEMPT_RT must change > > task state if the task blocks during acquisition. Therefore, the > > corresponding lock wakeup restores the task state. Note that regular > > (not lock related) wakeups do not restore task state. > > - Task state is preserved across spinlock acquisition, ensuring that the > task-state rules apply to all kernel configurations. Non-PREEMPT_RT > kernels leave task state untouched. However, PREEMPT_RT must change > task state if the task blocks during acquisition. Therefore, it > saves the current task state before blocking and the corresponding > lock wakeup restores it. A regular not lock related wakeup sets the > task state to RUNNING. If this happens while the task is blocked on > a spinlock then the saved task state is changed so that correct > state is restored on lock wakeup. > > Hmm? I of course cannot resist editing the last two sentences: ... Other types of wakeups unconditionally set task state to RUNNING. If this happens while a task is blocked while acquiring a spinlock, then the task state is restored to its pre-acquisition value at lock-wakeup time. > > But this code failes on PREEMPT_RT kernels because the memory allocator > > is fully preemptible and therefore cannot be invoked from truly atomic > > contexts. However, it is perfectly fine to invoke the memory allocator > > while holding a normal non-raw spinlocks because they do not disable > > preemption:: > > > >> + spin_lock(&lock); > >> + p = kmalloc(sizeof(*p), GFP_ATOMIC); > >> + > >> +Most places which use GFP_ATOMIC allocations are safe on PREEMPT_RT as the > >> +execution is forced into thread context and the lock substitution is > >> +ensuring preemptibility. > > > > Interestingly enough, most uses of GFP_ATOMIC allocations are > > actually safe on PREEMPT_RT because the the lock substitution ensures > > preemptibility. Only those GFP_ATOMIC allocations that are invoke > > while holding a raw spinlock or with preemption otherwise disabled need > > adjustment to work correctly on PREEMPT_RT. > > > > [ I am not as confident of the above as I would like to be... ] > > I'd leave that whole paragraph out. This documents the rules and from > the above code examples it's pretty clear what works and what not :) Works for me! ;-) > > And meeting time, will continue later! > > Enjoy! Not bad, actually, as meetings go. Thanx, Paul
