linux-kernel - Re: [PATCH] mm/swap: piggyback lru_add_drain

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Date:   Fri, 4 Oct 2019 05:10:17 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls

On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> This is very slow operation. There is no reason to do it again if somebody
> else already drained all per-cpu vectors after we waited for lock.
> +	seq = raw_read_seqcount_latch(&seqcount);
> +
>  	mutex_lock(&lock);
> +
> +	/* Piggyback on drain done by somebody else. */
> +	if (__read_seqcount_retry(&seqcount, seq))
> +		goto done;
> +
> +	raw_write_seqcount_latch(&seqcount);
> +

Do we really need the seqcount to do this?  Wouldn't a mutex_trylock()
have the same effect?