Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH v2] locking/qspinlock: Do spin-wait in slowpath if preemptible | Date | Tue, 20 Sep 2022 15:55:42 -0400 |
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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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