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Date: Fri, 31 Jan 2025 14:55:36 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking
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In-Reply-To: <20250131090658.3386285-2-senozhatsky@chromium.org>
References: <20250131090658.3386285-1-senozhatsky@chromium.org>
	<20250131090658.3386285-2-senozhatsky@chromium.org>
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On Fri, 31 Jan 2025 18:06:00 +0900 Sergey Senozhatsky <senozhatsky@chromium.org> wrote:

> +static void zram_slot_write_lock(struct zram *zram, u32 index)
> +{
> +	atomic_t *lock = &zram->table[index].lock;
> +	int old = atomic_read(lock);
> +
> +	do {
> +		if (old != ZRAM_ENTRY_UNLOCKED) {
> +			cond_resched();
> +			old = atomic_read(lock);
> +			continue;
> +		}
> +	} while (!atomic_try_cmpxchg(lock, &old, ZRAM_ENTRY_WRLOCKED));
> +}

I expect that if the calling userspace process has realtime policy (eg
SCHED_FIFO) then the cond_resched() won't schedule SCHED_NORMAL tasks
and this becomes a busy loop.  And if the machine is single-CPU, the
loop is infinite.

I do agree that for inventing new locking schemes, the bar is set
really high.