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!
Oleg.
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2049,6 +2049,9 @@ static int prctl_get_tid_address(struct task_struct *me, int __user **tid_addr)
}
#endif
+#include <linux/delay.h>
+
+
SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
unsigned long, arg4, unsigned long, arg5)
{
@@ -2062,6 +2065,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
error = 0;
switch (option) {
+ case 666:
+ preempt_disable();
+ pr_crit("SPIN start\n");
+ while (arg2--)
+ mdelay(1);
+ pr_crit("SPIN done!\n");
+ preempt_enable();
+ break;
+ case 777:
+ pr_crit("SYNC start\n");
+ synchronize_rcu();
+ pr_crit("SYNC done!\n");
+ break;
case PR_SET_PDEATHSIG:
if (!valid_signal(arg2)) {
error = -EINVAL;
--
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