linux-kernel - Re: [PATCH 4.19 02/80] shmem: fix possible deadlocks on shmlock_user

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Date:   Mon, 18 May 2020 23:13:30 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        syzbot+c8a8197c8852f566b9d9@...kaller.appspotmail.com,
        syzbot+40b71e145e73f78f81ad@...kaller.appspotmail.com,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 02/80] shmem: fix possible deadlocks on
 shmlock_user_lock

Hi!

> This may not risk an actual deadlock, since shmem inodes do not take
> part in writeback accounting, but there are several easy ways to avoid
> it.

...

> Take info->lock out of the chain and the possibility of deadlock or
> lockdep warning goes away.

It is unclear to me if actual possibility of deadlock exists or not,
but anyway:

>  	int retval = -ENOMEM;
>  
> -	spin_lock_irq(&info->lock);
> +	/*
> +	 * What serializes the accesses to info->flags?
> +	 * ipc_lock_object() when called from shmctl_do_lock(),
> +	 * no serialization needed when called from shm_destroy().
> +	 */
>  	if (lock && !(info->flags & VM_LOCKED)) {
>  		if (!user_shm_lock(inode->i_size, user))
>  			goto out_nomem;

Should we have READ_ONCE() here? If it is okay, are concurency
sanitizers smart enough to realize that it is okay? Replacing warning
with different one would not be exactly a win...

Best regards,

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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