Messages in this thread Patch in this message | | | Date | Mon, 29 Oct 2018 07:27:35 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt quiescent states when disabled |
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On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote: > Hi Paul and all, > > > -----Original Message----- > > From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel- > > owner@vger.kernel.org] On Behalf Of Paul E. McKenney > > Sent: Thursday, August 30, 2018 01:21 > > To: linux-kernel@vger.kernel.org > > Cc: mingo@kernel.org; jiangshanlai@gmail.com; dipankar@in.ibm.com; > > akpm@linux-foundation.org; mathieu.desnoyers@efficios.com; > > josh@joshtriplett.org; tglx@linutronix.de; peterz@infradead.org; > > rostedt@goodmis.org; dhowells@redhat.com; edumazet@google.com; > > fweisbec@gmail.com; oleg@redhat.com; joel@joelfernandes.org; Paul E. > > McKenney <paulmck@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 <paulmck@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?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index bd8186d0f4a7..f1b40c6d36e4 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -424,7 +424,7 @@ void __rcu_read_unlock(void) --t->rcu_read_lock_nesting; } else { barrier(); /* critical section before exit code. */ - t->rcu_read_lock_nesting = INT_MIN; + t->rcu_read_lock_nesting = -INT_MAX; barrier(); /* assign before ->rcu_read_unlock_special load */ if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) rcu_read_unlock_special(t); @@ -617,11 +617,11 @@ static void rcu_preempt_deferred_qs(struct task_struct *t) if (!rcu_preempt_need_deferred_qs(t)) return; if (couldrecurse) - t->rcu_read_lock_nesting -= INT_MIN; + t->rcu_read_lock_nesting -= INT_MAX; local_irq_save(flags); rcu_preempt_deferred_qs_irqrestore(t, flags); if (couldrecurse) - t->rcu_read_lock_nesting += INT_MIN; + t->rcu_read_lock_nesting += INT_MAX; } /*
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