[PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible

[PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible

[Waiman Long]

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

Re: [PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible

[Waiman Long]

On 9/20/22 15:55, Waiman Long wrote:
> 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>

On second thought, I believe the proper way to fix this is to make sure 
that all the callers of arch_spin_lock() has preemption properly 
disabled. Will work on another patch set to do that. So please ignore 
this patch and sorry for the noise.

Cheers,
Longman

Re: [PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible

[Peter Zijlstra]

On Tue, Sep 20, 2022 at 03:55:42PM -0400, Waiman Long wrote:
> 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

Using arch_spin_lock() without disabling preemption is a straight up
bug. Don't work around that.