linux-kernel - Re: [PATCH 02/10] staging: erofs: fix race when the managed cache is enabled

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Date:   Thu, 22 Nov 2018 11:17:11 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     devel@...verdev.osuosl.org, linux-erofs@...ts.ozlabs.org,
        Chao Yu <yuchao0@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>, weidu.du@...wei.com,
        Miao Xie <miaoxie@...wei.com>
Subject: Re: [PATCH 02/10] staging: erofs: fix race when the managed cache is
 enabled

On Tue, Nov 20, 2018 at 10:34:17PM +0800, Gao Xiang wrote:
> When the managed cache is enabled, the last reference count
> of a workgroup must be used for its workstation.
> 
> Otherwise, it could lead to incorrect (un)freezes in
> the reclaim path, and it would be harmful.
> 
> A typical race as follows:
> 
> Thread 1 (In the reclaim path)  Thread 2
> workgroup_freeze(grp, 1)                                refcnt = 1
> ...
> workgroup_unfreeze(grp, 1)                              refcnt = 1
>                                 workgroup_get(grp)      refcnt = 2 (x)
> workgroup_put(grp)                                      refcnt = 1 (x)
>                                 ...unexpected behaviors
> 
> * grp is detached but still used, which violates cache-managed
>   freeze constraint.
> 
> Reviewed-by: Chao Yu <yuchao0@...wei.com>
> Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
> ---
>  drivers/staging/erofs/internal.h |   1 +
>  drivers/staging/erofs/utils.c    | 131 +++++++++++++++++++++++++++------------
>  2 files changed, 93 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/staging/erofs/internal.h b/drivers/staging/erofs/internal.h
> index 57575c7f5635..89dbd0888e53 100644
> --- a/drivers/staging/erofs/internal.h
> +++ b/drivers/staging/erofs/internal.h
> @@ -250,6 +250,7 @@ static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
>  }
>  
>  #define __erofs_workgroup_get(grp)	atomic_inc(&(grp)->refcount)
> +#define __erofs_workgroup_put(grp)	atomic_dec(&(grp)->refcount)

Any specific reason why you are not using the refcount.h api instead of
"doing it yourself" with atomic_inc/dec()?

I'm not rejecting this, just curious.

thanks,

greg k-h