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Subject[PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible
Date
There are some code paths in the kernel where arch_spin_lock() will be
called directly when the lock isn't expected to be contended and critical
section is short. For example, tracing_saved_cmdlines_size_read()
in kernel/trace/trace.c does that.

In most cases, preemption is also not disabled. This creates a problem
for the qspinlock slowpath which expects preemption to be disabled
to guarantee the safe use of per cpu qnodes structure. To work around
these special use cases, add a preemption count check in the slowpath
and do a simple spin-wait when preemption isn't disabled.

Fixes: a33fda35e3a7 ("Introduce a simple generic 4-byte queued spinlock")
Signed-off-by: Waiman Long <longman@redhat.com>
---
[v2] Move down spin-wait to after the pending bit wait.

kernel/locking/qspinlock.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 65a9a10caa6f..d0159038084d 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -398,6 +398,23 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
queue:
lockevent_inc(lock_slowpath);
pv_queue:
+#ifdef CONFIG_PREEMPT_COUNT
+ /*
+ * As arch_spin_lock() can be called directly in some use cases
+ * where the lock isn't expected to be contended, critical section
+ * is short and preemption isn't disabled, we can't use qnodes in
+ * this case as state may be screwed up in case preemption happens
+ * or preemption warning may be printed (CONFIG_DEBUG_PREEMPT).
+ * Just do a simple spin-wait in this case as the lock shouldn't be
+ * contended for long.
+ */
+ if (unlikely(!preempt_count())) {
+ while (!queued_spin_trylock(lock))
+ cpu_relax();
+ return;
+ }
+#endif
+
node = this_cpu_ptr(&qnodes[0].mcs);
idx = node->count++;
tail = encode_tail(smp_processor_id(), idx);
--
2.31.1
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