linux-kernel - Re: [PATCH v7] zswap: replace RB tree with xarray

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Message-ID: <Zfs9HjKzKlw6qGjb@google.com>
Date: Wed, 20 Mar 2024 19:46:38 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chris Li <chrisl@...nel.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Nhat Pham <nphamcs@...il.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Chengming Zhou <zhouchengming@...edance.com>, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v7] zswap: replace RB tree with xarray

On Wed, Mar 20, 2024 at 12:41:48PM -0700, Chris Li wrote:
> On Wed, Mar 20, 2024 at 12:34 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > > you can expect it to be coherent.
> >
> > Fair enough, but don't we still need a barrier there? Couldn't some
> > initializations still be reorder after zswap_lru_add()?
> 
> I am under the impression that the lru list handling function already
> handles the barrier. If an explicit barrier is needed, wouldn't all
> other lru callers have barriers sprinkling all over the place?

list_lru_add() holds a lock, which implies a compiler barrier. I was
wondering if we need smp_wmb() to prevent other CPUs from observing a
partially initialized entry during writeback.

I am not sure if the list lru makes such assumptions about the need for
barriers.