On Tue, Oct 30, 2018 at 05:58:00AM -0700, Paul E. McKenney wrote: > On Mon, Oct 29, 2018 at 08:44:52PM -0700, Joel Fernandes wrote: > > On Mon, Oct 29, 2018 at 07:27:35AM -0700, Paul E. McKenney wrote: > > > On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote: > > > > Hi Paul and all, > > > > > > > > > -----Original Message----- > > > > > From: linux-kernel-ow...@vger.kernel.org [mailto:linux-kernel- > > > > > ow...@vger.kernel.org] On Behalf Of Paul E. McKenney > > > > > Sent: Thursday, August 30, 2018 01:21 > > > > > To: linux-kernel@vger.kernel.org > > > > > Cc: mi...@kernel.org; jiangshan...@gmail.com; dipan...@in.ibm.com; > > > > > a...@linux-foundation.org; mathieu.desnoy...@efficios.com; > > > > > j...@joshtriplett.org; t...@linutronix.de; pet...@infradead.org; > > > > > rost...@goodmis.org; dhowe...@redhat.com; eduma...@google.com; > > > > > fweis...@gmail.com; o...@redhat.com; j...@joelfernandes.org; Paul E. > > > > > McKenney <paul...@linux.vnet.ibm.com> > > > > > Subject: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt > > > > > quiescent states when disabled > > > > > > > > > > This commit defers reporting of RCU-preempt quiescent states at > > > > > rcu_read_unlock_special() time when any of interrupts, softirq, or > > > > > preemption are disabled. These deferred quiescent states are > > > > > reported at a > > > > > later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug offline > > > > > operation. Of course, if another RCU read-side critical section has > > > > > started in > > > > > the meantime, the reporting of the quiescent state will be further > > > > > deferred. > > > > > > > > > > This also means that disabling preemption, interrupts, and/or > > > > > softirqs will act > > > > > as an RCU-preempt read-side critical section. > > > > > This is enforced by checking preempt_count() as needed. > > > > > > > > > > Some special cases must be handled on an ad-hoc basis, for example, > > > > > context switch is a quiescent state even though both the scheduler and > > > > > do_exit() disable preemption. In these cases, additional calls to > > > > > rcu_preempt_deferred_qs() override the preemption disabling. Similar > > > > > logic > > > > > overrides disabled interrupts in rcu_preempt_check_callbacks() > > > > > because in > > > > > this case the quiescent state happened just before the corresponding > > > > > scheduling-clock interrupt. > > > > > > > > > > In theory, this change lifts a long-standing restriction that > > > > > required that if > > > > > interrupts were disabled across a call to rcu_read_unlock() that the > > > > > matching > > > > > rcu_read_lock() also be contained within that interrupts-disabled > > > > > region of > > > > > code. Because the reporting of the corresponding RCU-preempt > > > > > quiescent > > > > > state is now deferred until after interrupts have been enabled, it is > > > > > no longer > > > > > possible for this situation to result in deadlocks involving the > > > > > scheduler's > > > > > runqueue and priority-inheritance locks. This may allow some code > > > > > simplification that might reduce interrupt latency a bit. > > > > > Unfortunately, in > > > > > practice this would also defer deboosting a low-priority task that > > > > > had been > > > > > subjected to RCU priority boosting, so real-time-response > > > > > considerations > > > > > might well force this restriction to remain in place. > > > > > > > > > > Because RCU-preempt grace periods are now blocked not only by RCU > > > > > read- > > > > > side critical sections, but also by disabling of interrupts, > > > > > preemption, and > > > > > softirqs, it will be possible to eliminate RCU-bh and RCU-sched in > > > > > favor of > > > > > RCU-preempt in CONFIG_PREEMPT=y kernels. This may require some > > > > > additional plumbing to provide the network denial-of-service > > > > > guarantees > > > > > that have been traditionally provided by RCU-bh. Once these are in > > > > > place, > > > > > CONFIG_PREEMPT=n kernels will be able to fold RCU-bh into RCU-sched. > > > > > This would mean that all kernels would have but one flavor of RCU, > > > > > which > > > > > would open the door to significant code cleanup. > > > > > > > > > > Moving to a single flavor of RCU would also have the beneficial > > > > > effect of > > > > > reducing the NOCB kthreads by at least a factor of two. > > > > > > > > > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> [ > > > > > paulmck: > > > > > Apply rcu_read_unlock_special() preempt_count() feedback > > > > > from Joel Fernandes. ] > > > > > [ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in > > > > > response to bug reports from kbuild test robot. ] [ paulmck: Fix > > > > > bug located > > > > > by kbuild test robot involving recursion > > > > > via rcu_preempt_deferred_qs(). ] > > > > > --- > > > > > .../RCU/Design/Requirements/Requirements.html | 50 +++--- > > > > > include/linux/rcutiny.h | 5 + > > > > > kernel/rcu/tree.c | 9 ++ > > > > > kernel/rcu/tree.h | 3 + > > > > > kernel/rcu/tree_exp.h | 71 +++++++-- > > > > > kernel/rcu/tree_plugin.h | 144 > > > > > +++++++++++++----- > > > > > 6 files changed, 205 insertions(+), 77 deletions(-) > > > > > > > > > > > > > We started seeing the trace below in our regression system, after I > > > > bisected I found this is the offending commit. > > > > This appears immediately on boot. > > > > Please let me know if you need any additional details. > > > > > > Interesting. Here is the offending function: > > > > > > static void rcu_preempt_deferred_qs(struct task_struct *t) > > > { > > > unsigned long flags; > > > bool couldrecurse = t->rcu_read_lock_nesting >= 0; > > > > > > if (!rcu_preempt_need_deferred_qs(t)) > > > return; > > > if (couldrecurse) > > > t->rcu_read_lock_nesting -= INT_MIN; > > > local_irq_save(flags); > > > rcu_preempt_deferred_qs_irqrestore(t, flags); > > > if (couldrecurse) > > > t->rcu_read_lock_nesting += INT_MIN; > > > } > > > > > > Using twos-complement arithmetic (which the kernel build gcc arguments > > > enforce, last I checked) this does work. But as UBSAN says, subtracting > > > INT_MIN is unconditionally undefined behavior according to the C standard. > > > > > > Good catch!!! > > > > > > So how do I make the above code not simply function, but rather meet > > > the C standard? > > > > > > One approach to add INT_MIN going in, then add INT_MAX and then add 1 > > > coming out. > > > > > > Another approach is to sacrifice the INT_MAX value (should be plenty > > > safe), thus subtract INT_MAX going in and add INT_MAX coming out. > > > For consistency, I suppose that I should change the INT_MIN in > > > __rcu_read_unlock() to -INT_MAX. > > > > > > I could also leave __rcu_read_unlock() alone and XOR the top > > > bit of t->rcu_read_lock_nesting on entry and exit to/from > > > rcu_preempt_deferred_qs(). > > > > > > Sacrificing the INT_MIN value seems most maintainable, as in the following > > > patch. Thoughts? > > > > The INT_MAX naming could be very confusing for nesting levels, could we not > > instead just define something like: > > #define RCU_NESTING_MIN (INT_MIN - 1) > > #define RCU_NESTING_MAX (INT_MAX) > > > > and just use that? also one more comment below: > > Hmmm... There is currently no use for RCU_NESTING_MAX, but if the check > at the end of __rcu_read_unlock() were to be extended to check for > too-deep positive nesting, it would need to check for something like > INT_MAX/2. You could of course argue that the current check against > INT_MIN/2 should instead be against -INT_MAX/2, but there really isn't > much difference between the two. > > Another approach would be to convert to unsigned in order to avoid the > overflow problem completely. > > For the moment, anyway, I am inclined to leave it as is.
Both the unsigned and INT_MIN/2 options sound good to me, but if you want leave it as is, that would be fine as well. thanks, - Joel