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From: David Sterba <dsterba@suse.cz>
To: Anand Jain <anand.jain@oracle.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v4 1/3] btrfs: scrub: fix circular locking dependency warning
Date: Wed, 30 Jan 2019 15:07:58 +0100
Message-ID: <20190130140758.GR2900@twin.jikos.cz> (raw)
In-Reply-To: <1548830702-14676-2-git-send-email-anand.jain@oracle.com>

On Wed, Jan 30, 2019 at 02:45:00PM +0800, Anand Jain wrote:
> v3->v4: Fix list corruption as reported by btrfs/073 by David.
>            [1]
>            https://patchwork.kernel.org/patch/10705741/
> 	Which I was able to reproduce with an instrumented kernel but not with
> 	btrfs/073.
>         In v3 patch, it releases the fs_info::scrub_lock to destroy the work queue
> 	which raced with new scrub requests, overwriting the scrub workers
> 	pointers. So in v4, it kills the function scrub_workers_put(), and
> 	performs the destroy_workqueue in two stages, with worker pointers
> 	copied locally.

> @@ -3932,9 +3925,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  
>  	mutex_lock(&fs_info->scrub_lock);
>  	dev->scrub_ctx = NULL;
> -	scrub_workers_put(fs_info);
> +	if (--fs_info->scrub_workers_refcnt == 0) {
> +		scrub_workers = fs_info->scrub_workers;
> +		scrub_wr_comp = fs_info->scrub_wr_completion_workers;
> +		scrub_parity = fs_info->scrub_parity_workers;
> +	}
>  	mutex_unlock(&fs_info->scrub_lock);
>  
> +	btrfs_destroy_workqueue(scrub_workers);
> +	btrfs_destroy_workqueue(scrub_wr_comp);
> +	btrfs_destroy_workqueue(scrub_parity);

https://lore.kernel.org/linux-btrfs/1543554924-17397-2-git-send-email-anand.jain@oracle.com/

Comparing to the previous version, it's almost the same I think. If
scrub_workers_get races between the unlock and destroy_workers, anything
that uses fs_info->scrub_wokers will soon use freed memory.

The difference is that the worker pointers are read from fs_info under a
lock but are still used outside. I haven't tested this version but from
the analysis of previous crash, I don't see how v4 is supposed to be
better.

  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-30  6:44 [PATCH v4 0/3] btrfs: scrub: fix scrub_lock Anand Jain
2019-01-30  6:45 ` [PATCH v4 1/3] btrfs: scrub: fix circular locking dependency warning Anand Jain
2019-01-30 14:07   ` David Sterba [this message]
2019-01-31  6:34     ` Anand Jain
2019-02-12 15:49       ` David Sterba
2019-02-12 16:45   ` David Sterba
2019-02-13 16:02     ` Anand Jain
2019-01-30  6:45 ` [PATCH v4 2/3] btrfs: scrub: add scrub_lock lockdep check in scrub_workers_get Anand Jain
2019-01-30  6:45 ` [PATCH v4 3/3] btrfs: scrub: convert scrub_workers_refcnt to refcount_t Anand Jain
2019-02-08 17:02 ` [PATCH v4 0/3] btrfs: scrub: fix scrub_lock David Sterba
2019-02-11  8:09   ` Anand Jain
2019-02-11  8:38 ` [PATCH v5 3/3] btrfs: scrub: convert scrub_workers_refcnt to refcount_t Anand Jain
2019-02-12 17:15 ` [PATCH] btrfs: scrub: add assertions for worker pointers David Sterba
2019-02-13 16:02   ` Anand Jain
2019-02-13 17:35 ` [PATCH v4 0/3] btrfs: scrub: fix scrub_lock David Sterba

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