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Date:   Fri, 23 Nov 2018 10:51:32 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
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        linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        boqun.feng@gmail.com
Subject: Re: [PATCH v2] kmemleak: Turn kmemleak_lock to raw spinlock on RT
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On Fri, 23 Nov 2018 11:31:31 +0000
Catalin Marinas <catalin.marinas@arm.com> wrote:

> With qwrlocks, the readers will normally block if there is a pending
> writer (to avoid starving the writer), unless in_interrupt() when the
> readers are allowed to starve a pending writer.
> 
> TLA+/PlusCal model here:  ;)
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/tree/qrwlock.tla
> 


And the code appears to confirm it too:

void queued_read_lock_slowpath(struct qrwlock *lock)
{
	/*
	 * Readers come here when they cannot get the lock without waiting
	 */
	if (unlikely(in_interrupt())) {
		/*
		 * Readers in interrupt context will get the lock immediately
		 * if the writer is just waiting (not holding the lock yet),
		 * so spin with ACQUIRE semantics until the lock is available
		 * without waiting in the queue.
		 */
		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
		return;
	}
	atomic_sub(_QR_BIAS, &lock->cnts);

	/*
	 * Put the reader into the wait queue
	 */
	arch_spin_lock(&lock->wait_lock);
	atomic_add(_QR_BIAS, &lock->cnts);


-- Steve