* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Sun, Jun 14, 2015 at 09:38:25PM +0200, Oleg Nesterov wrote: > > On 06/14, Oleg Nesterov wrote: > > > > > > 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(). > > > > > > I've even checked this ;) I applied the stupid patch below and then > > > > $ taskset 2 perl -e 'syscall 157, 666, 5000' & > > [1] 565 > > > > $ taskset 1 perl -e 'syscall 157, 777' > > > > $ > > [1]+ Done taskset 2 perl -e 'syscall 157, 666, 5000' > > > > $ dmesg -c > > SPIN start > > SYNC start > > SYNC done! > > SPIN done! > > Please accept my apologies for my late entry to this thread. > Youngest kid graduated from university this weekend, so my > attention has been elsewhere. Congratulations! :-) > If you were to disable interrupts instead of preemption, I would expect > that the preemptible-RCU grace period would be blocked -- though I am > not particularly comfortable with people relying on disabled interrupts > blocking a preemptible-RCU grace period. > > Here is what can happen if you try to block a preemptible-RCU grace > period by disabling preemption, assuming that there are at least two > online CPUs in the system: > > 1. CPU 0 does spin_lock(), which disables preemption. > > 2. CPU 1 starts a grace period. > > 3. CPU 0 takes a scheduling-clock interrupt. It raises softirq, > and the RCU_SOFTIRQ handler notes that there is a new grace > period and sets state so that a subsequent quiescent state on > this CPU will be noted. > > 4. CPU 0 takes another scheduling-clock interrupt, which checks > current->rcu_read_lock_nesting, and notes that there is no > preemptible-RCU read-side critical section in progress. It > again raises softirq, and the RCU_SOFTIRQ handler reports > the quiescent state to core RCU. > > 5. Once each of the other CPUs report a quiescent state, the > grace period can end, despite CPU 0 having preemption > disabled the whole time. > > So Oleg's test is correct, disabling preemption is not sufficient > to block a preemptible-RCU grace period. I stand corrected! > The usual suggestion would be to add rcu_read_lock() just after the lock is > acquired and rcu_read_unlock() just before each release of that same lock. Will fix it that way. Thanks, Ingo -- 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>