From d682b596d99345ef0000e7017db714ba7f29e017 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Tue, 29 Jan 2019 22:53:45 +0100 Subject: [PATCH] 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 Signed-off-by: Waiman Long Signed-off-by: Peter Zijlstra (Intel) Acked-by: Will Deacon Cc: Andrew Morton Cc: Borislav Petkov Cc: H. Peter Anvin Cc: James Morse Cc: Linus Torvalds Cc: Paul E. McKenney Cc: SRINIVAS Cc: Thomas Gleixner Cc: Zhenzhong Duan Link: https://lkml.kernel.org/r/1548798828-16156-2-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar --- kernel/locking/qspinlock.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 8a8c3c208c5e4b..0875053c4050b3 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -412,6 +412,21 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) 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); /*