linux-kernel - Re: [PATCH v2 5/6] locking/rtmutex: Use rt

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Date:   Sun, 3 Sep 2023 11:54:41 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        boqun.feng@...il.com, bristot@...hat.com, bsegall@...gle.com,
        dietmar.eggemann@....com, jstultz@...gle.com,
        juri.lelli@...hat.com, longman@...hat.com, mgorman@...e.de,
        mingo@...hat.com, rostedt@...dmis.org, swood@...hat.com,
        vincent.guittot@...aro.org, vschneid@...hat.com, will@...nel.org
Subject: Re: [PATCH v2 5/6] locking/rtmutex: Use rt_mutex specific scheduler
 helpers

On Fri, Aug 25, 2023 at 08:10:32PM +0200, Sebastian Andrzej Siewior wrote:
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -71,6 +71,7 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
>  	struct rt_mutex_base *rtm = &rwb->rtmutex;
>  	int ret;
>  
> +	rwbase_pre_schedule();
>  	raw_spin_lock_irq(&rtm->wait_lock);
>  
>  	/*
> @@ -125,6 +126,7 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
>  		rwbase_rtmutex_unlock(rtm);
>  
>  	trace_contention_end(rwb, ret);
> +	rwbase_post_schedule();
>  	return ret;
>  }
>  
> @@ -237,6 +239,8 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  	/* Force readers into slow path */
>  	atomic_sub(READER_BIAS, &rwb->readers);
>  
> +	rt_mutex_pre_schedule();
> +
>  	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>  	if (__rwbase_write_trylock(rwb))
>  		goto out_unlock;
> @@ -248,6 +252,7 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  		if (rwbase_signal_pending_state(state, current)) {
>  			rwbase_restore_current_state();
>  			__rwbase_write_unlock(rwb, 0, flags);
> +			rt_mutex_post_schedule();
>  			trace_contention_end(rwb, -EINTR);
>  			return -EINTR;
>  		}
> @@ -266,6 +271,7 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  
>  out_unlock:
>  	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> +	rt_mutex_post_schedule();
>  	return 0;
>  }

Shouldn't rwbase_write_lock() use rwbase_{pre|post}_schedule()?

With this change as-is, I observe deadlocks due to lock recursion from
write_lock() specifically, because write_lock() ends up flushing block requests.

Sultan