linux-kernel - Re: [PATCH v3 6/6] pid: drop irq disablement around pidmap

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Message-ID: <20250201174544.GB26042@redhat.com>
Date: Sat, 1 Feb 2025 18:45:44 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: ebiederm@...ssion.com, brauner@...nel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 6/6] pid: drop irq disablement around pidmap_lock

On 02/01, Oleg Nesterov wrote:
>
> However, shouldn't this patch also remove the comment which explains
> the possible lock inversion? Above put_pid(),
>
> 	/*
> 	 * Note: disable interrupts while the pidmap_lock is held as an
> 	 * interrupt might come in and do read_lock(&tasklist_lock).
> 	 *
> 	 * If we don't disable interrupts there is a nasty deadlock between
> 	 * detach_pid()->free_pid() and another cpu that does
> 	 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
> 	 * read_lock(&tasklist_lock);
> 	 *
> 	 * After we clean up the tasklist_lock and know there are no
> 	 * irq handlers that take it we can leave the interrupts enabled.
> 	 * For now it is easier to be safe than to prove it can't happen.
> 	 */

Ah, sorry, please forget, you did it in the previous patch. which probably
needs some more discussion...

Oleg.