Commit d682b596 authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar

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: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarWill 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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 07879c6a
...@@ -412,6 +412,21 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) ...@@ -412,6 +412,21 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
idx = node->count++; idx = node->count++;
tail = encode_tail(smp_processor_id(), idx); 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); node = grab_mcs_node(node, idx);
/* /*
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment