linux-kernel - Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

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Date:   Fri, 20 Jan 2023 14:38:27 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linux-RT <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

On 2023-01-20 13:24:41 [+0000], Mel Gorman wrote:
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -264,12 +291,17 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
>  		if (__rwbase_write_trylock(rwb))
>  			break;
>  
> +		/* Record timeout when reader bias is ignored. */
> +		rwb->waiter_timeout = jiffies + RWBASE_RT_WAIT_TIMEOUT;
		rwb->waiter_timeout = (jiffies + RWBASE_RT_WAIT_TIMEOUT) | 1;

There is the unlikely case that (jiffies + RWBASE_RT_WAIT_TIMEOUT) = 0
on 32bit where it is not jiffies64.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

> +
>  		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
>  		rwbase_schedule();
>  		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
>  
>  		set_current_state(state);
>  	}
> +
> +	rwb->waiter_timeout = 0;
>  	rwbase_restore_current_state();
>  	trace_contention_end(rwb, 0);

Sebastian