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>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 8a8c3c208c5e4b0f05694c93b177ff7f4fac5ee7..0875053c4050b32aed9581e90b4338685a9461f1 100644 (file)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -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);
 
        /*