linux-kernel - Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

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Message-ID: <ZaHmQU5DouedI9kS@tassilo>
Date: Fri, 12 Jan 2024 17:24:17 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Marco Elver <elver@...gle.com>
Cc: Andrey Konovalov <andreyknvl@...il.com>,
	Oscar Salvador <osalvador@...e.de>, andrey.konovalov@...ux.dev,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Potapenko <glider@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
	Evgenii Stepanov <eugenis@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

On Fri, Jan 12, 2024 at 11:15:05PM +0100, Marco Elver wrote:
> +		/*
> +		 * Stack traces of size 0 are never saved, and we can simply use
> +		 * the size field as an indicator if this is a new unused stack
> +		 * record in the freelist.
> +		 */
> +		stack->size = 0;

I would use WRITE_ONCE here too, at least for TSan.

> +		return NULL;
> +
> +	/*
> +	 * We maintain the invariant that the elements in front are least
> +	 * recently used, and are therefore more likely to be associated with an
> +	 * RCU grace period in the past. Consequently it is sufficient to only
> +	 * check the first entry.
> +	 */
> +	stack = list_first_entry(&free_stacks, struct stack_record, free_list);
> +	if (stack->size && !poll_state_synchronize_rcu(stack->rcu_state))

READ_ONCE (also for TSan, and might be safer long term in case the
compiler considers some fancy code transformation)

> +		return NULL;
>  
> +		stack = depot_pop_free();
> +		if (WARN_ON(!stack))

Won't you get nesting problems here if this triggers due to the print?
I assume the nmi safe printk won't consider it like an NMI.

>  	counters[DEPOT_COUNTER_FREELIST_SIZE]++;
>  	counters[DEPOT_COUNTER_FREES]++;
>  	counters[DEPOT_COUNTER_INUSE]--;
> +
> +	printk_deferred_exit();

Ah this handles the WARN_ON? Should be ok then.

-Andi