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On Thu, 12 Feb 2026 07:59:17 +0800, Hillf Danton wrote: > Nope mb is not enough. > > int qd = atomic_read(&q->quiesce_depth); > for (;;) { > int v = qd + 1; > if (atomic_try_cmpxchg(&q->quiesce_depth, &qd, v)) > break; > } atomic_inc() is already unconditionally atomic and does exactly the same as the cmpxchg loop above - it atomically increments the value by one. The smp_mb__after_atomic() following it provides the necessary memory ordering to ensure the store is visible before subsequent loads in blk_mq_run_hw_queue(). Could you clarify what specific scenario you see where this would be insufficient? > More important however, why not send this patch to the RT tree instead, > given the spin lock is good and correct in mainline. By good I mean > fixing RT stuff in the mainline is not encouraged. PREEMPT_RT has been merged into mainline since v6.12. There is no separate RT tree for this kind of fix anymore. The spinlock in question converts to rt_mutex under PREEMPT_RT, which is now a mainline configuration option, so the contention issue is a mainline problem. Beyond the RT aspect, replacing the spinlock with atomic_t in the hot path is arguably a simplification that benefits all configurations - it removes locking overhead and eliminates the two-variable synchronization (quiesce_depth + QUEUE_FLAG_QUIESCED) in favor of a single atomic counter. Best regards, Ionut