linux-kernel - Re: [PATCH] mm: memory-failure: use rcu lock instead of tasklist_lock when collect

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZOLpbwwk4esztLaO@casper.infradead.org>
Date:   Mon, 21 Aug 2023 05:34:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Tong Tiangen <tongtiangen@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, wangkefeng.wang@...wei.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memory-failure: use rcu lock instead of
 tasklist_lock when collect_procs()

On Mon, Aug 21, 2023 at 10:25:34AM +0800, Tong Tiangen wrote:
> +++ b/mm/memory-failure.c
> @@ -546,24 +546,26 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>   * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
>   * on behalf of the thread group. Return task_struct of the (first found)
>   * dedicated thread if found, and return NULL otherwise.
> - *
> - * We already hold read_lock(&tasklist_lock) in the caller, so we don't
> - * have to call rcu_read_lock/unlock() in this function.
>   */
>  static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
>  {
>  	struct task_struct *t;
>  
> +	rcu_read_lock();
>  	for_each_thread(tsk, t) {
>  		if (t->flags & PF_MCE_PROCESS) {
>  			if (t->flags & PF_MCE_EARLY)
> -				return t;
> +				goto found;
>  		} else {
>  			if (sysctl_memory_failure_early_kill)
> -				return t;
> +				goto found;
>  		}
>  	}
> -	return NULL;
> +
> +	t = NULL;
> +found:
> +	rcu_read_unlock();
> +	return t;
>  }

I don't understand why you need to modify find_early_kill_thread() at
all.  It's still true that the caller holds _a_ lock protecting it; the
comment needs to be updated to reflect that it might be the RCU lock
or the tasklist_lock (or did you change all callers?), but there's no
need for this function to take the RCU lock itself, afaics?