Commit d682b596 authored Jan 29, 2019 by Waiman Long Committed by Ingo Molnar Feb 04, 2019

locking/qspinlock: Handle > 4 slowpath nesting levels



Four queue nodes per CPU are allocated to enable up to 4 nesting levels
using the per-CPU nodes. Nested NMIs are possible in some architectures.
Still it is very unlikely that we will ever hit more than 4 nested
levels with contention in the slowpath.

When that rare condition happens, however, it is likely that the system
will hang or crash shortly after that. It is not good and we need to
handle this exception case.

This is done by spinning directly on the lock using repeated trylock.
This alternative code path should only be used when there is nested
NMIs. Assuming that the locks used by those NMI handlers will not be
heavily contended, a simple TAS locking should work out.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: James Morse <james.morse@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: SRINIVAS <srinivas.eeda@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Link: https://lkml.kernel.org/r/1548798828-16156-2-git-send-email-longman@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 07879c6a

kernel/locking/qspinlock.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -412,6 +412,21 @@ pv_queue:
		idx = node->count++;
		tail = encode_tail(smp_processor_id(), idx);

		/*
		* 4 nodes are allocated based on the assumption that there will
		* not be nested NMIs taking spinlocks. That may not be true in
		* some architectures even though the chance of needing more than
		* 4 nodes will still be extremely unlikely. When that happens,
		* we fall back to spinning on the lock directly without using
		* any MCS node. This is not the most elegant solution, but is
		* simple enough.
		*/
		if (unlikely(idx >= MAX_NODES)) {
		while (!queued_spin_trylock(lock))
		cpu_relax();
		goto release;
		}

		node = grab_mcs_node(node, idx);

		/*

Admin message