On 06/14, Ingo Molnar wrote: > > * Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > > + spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */ > > ^^^^^^^^^^^^^^^^^^^^^^^ > > > > Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand how it > > actually works ;) But, say, rcu_check_callbacks() can be called from irq and > > since spin_lock() doesn't increment current->rcu_read_lock_nesting this can lead > > to rcu_preempt_qs()? > > No, RCU grace periods are still defined by 'heavy' context boundaries such as > context switches, entering idle or user-space mode. > > PREEMPT_RCU is like traditional RCU, except that blocking is allowed within the > RCU read critical section - that is why it uses a separate nesting counter > (current->rcu_read_lock_nesting), not the preempt count. Yes. > But if a piece of kernel code is non-preemptible, such as a spinlocked region or > an irqs-off region, then those are still natural RCU read lock regions, regardless > of the RCU model, and need no additional RCU locking. I do not think so. Yes I understand that rcu_preempt_qs() itself doesn't finish the gp, but if there are no other rcu-read-lock holders then it seems synchronize_rcu() on another CPU can return _before_ spin_unlock(), this CPU no longer needs rcu_preempt_note_context_switch(). OK, I can be easily wrong, I do not really understand the implementation of PREEMPT_RCU. Perhaps preempt_disable() can actually act as rcu_read_lock() with the _current_ implementation. Still this doesn't look right even if happens to work, and Documentation/RCU/checklist.txt says: 11. Note that synchronize_rcu() -only- guarantees to wait until all currently executing rcu_read_lock()-protected RCU read-side critical sections complete. It does -not- necessarily guarantee that all currently running interrupts, NMIs, preempt_disable() code, or idle loops will complete. Therefore, if your read-side critical sections are protected by something other than rcu_read_lock(), do -not- use synchronize_rcu(). Oleg. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>