[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAJD7tkYp3GbuXV9G5bAZ1DetMmepV5ynciA+ukae7CKuxpXDJQ@mail.gmail.com>
Date: Wed, 12 Jun 2024 11:28:12 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Takero Funaki <flintglass@...il.com>
Cc: Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>,
Chengming Zhou <chengming.zhou@...ux.dev>, Jonathan Corbet <corbet@....net>,
Andrew Morton <akpm@...ux-foundation.org>,
Domenico Cerasuolo <cerasuolodomenico@...il.com>, linux-mm@...ck.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration
On Wed, Jun 12, 2024 at 11:16 AM Takero Funaki <flintglass@...il.com> wrote:
>
> 2024年6月12日(水) 3:26 Nhat Pham <nphamcs@...il.com>:
>
> >
> > As I have noted in v0, I think this is unnecessary and makes it more confusing.
> >
>
> Does spin_lock() ensure that compiler optimizations do not remove
> memory access to an external variable? I think we need to use
> READ_ONCE/WRITE_ONCE for shared variable access even under a spinlock.
> For example,
> https://elixir.bootlin.com/linux/latest/source/mm/mmu_notifier.c#L234
In this example, it seems like mmu_interval_set_seq() updates
interval_sub->invalidate_seq locklessly using WRITE_ONCE(). I think
this is why READ_ONCE() is required in that particular case.
>
> isn't this a common use case of READ_ONCE?
> ```c
> bool shared_flag = false;
> spinlock_t flag_lock;
>
> void somefunc(void) {
> for (;;) {
> spin_lock(&flag_lock);
> /* check external updates */
> if (READ_ONCE(shared_flag))
> break;
> /* do something */
> spin_unlock(&flag_lock);
> }
> spin_unlock(&flag_lock);
> }
> ```
> Without READ_ONCE, the check can be extracted from the loop by optimization.
According to Documentation/memory-barriers.txt, lock acquiring
functions are implicit memory barriers. Otherwise, the compiler would
be able to pull any memory access outside of the lock critical section
and locking wouldn't be reliable.
Powered by blists - more mailing lists