linux-kernel - Re: [PATCH] zsmalloc: do not use bit_spin

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Date:   Sun, 20 Dec 2020 02:56:18 +0100
From:   Mike Galbraith <efault@....de>
To:     Vitaly Wool <vitaly.wool@...sulko.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Cc:     Barry Song <song.bao.hua@...ilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Minchan Kim <minchan@...nel.org>,
        NitinGupta <ngupta@...are.org>
Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock

On Sun, 2020-12-20 at 02:22 +0200, Vitaly Wool wrote:
> zsmalloc takes bit spinlock in its _map() callback and releases it
> only in unmap() which is unsafe and leads to zswap complaining
> about scheduling in atomic context.
>
> To fix that and to improve RT properties of zsmalloc, remove that
> bit spinlock completely and use a bit flag instead.


> -static void pin_tag(unsigned long handle) __acquires(bitlock)
> +static void pin_tag(unsigned long handle)
>  {
> -	bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
> +	preempt_disable();
> +	while(test_and_set_bit(HANDLE_PIN_BIT, (unsigned long *)handle))
> +		cpu_relax();
> +	preempt_enable();
>  }

If try doesn't need to disable preemption, neither does pin.

	-Mike