linux-kernel - Re: [PATCH mm-unstable hotfix] mm/zswap: fix zswap_pools_lock usages after changing to percpu

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Message-ID: <1d920f2c-2f86-46b9-b5d9-b5615284e5c3@linux.dev>
Date: Wed, 28 Feb 2024 23:37:44 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, yosryahmed@...gle.com,
 nphamcs@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable hotfix] mm/zswap: fix zswap_pools_lock usages
 after changing to percpu_ref

On 2024/2/28 23:24, Matthew Wilcox wrote:
> On Wed, Feb 28, 2024 at 03:18:32PM +0000, Chengming Zhou wrote:
>> Now the release of zswap pool is controlled by percpu_ref, its release
>> callback (__zswap_pool_empty()) will be called when percpu_ref hit 0.
>> But this release callback may potentially be called from RCU callback
>> context by percpu_ref_kill(), which maybe in the interrupt context.
>>
>> So we need to use spin_lock_irqsave() and spin_unlock_irqrestore()
>> in the release callback: __zswap_pool_empty(). In other task context
>> places, spin_lock_irq() and spin_unlock_irq() are enough to avoid
>> potential deadlock.
> 
> RCU callback context is BH, not IRQ, so it's enough to use
> spin_lock_bh(), no?

You're right, it's the softirq context, so spin_lock_bh() is enough.

Thanks!