Date: Mon, 2 Feb 2015 15:52:51 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Fabian Frederick <fabf@skynet.be>
Cc: linux-kernel@vger.kernel.org, Evgeniy Dushistov <dushistov@mail.ru>
Subject: Re: [PATCH 1/1 linux-next] fs/ufs/super.c: remove
 CONFIG_SMP/CONFIG_PREEMPT testing
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References: <1422790617-7944-1-git-send-email-fabf@skynet.be>
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On Sun,  1 Feb 2015 12:36:57 +0100 Fabian Frederick <fabf@skynet.be> wrote:

> Let locking subsystem decide on mutex management.
> 
> ...
>
> --- a/fs/ufs/super.c
> +++ b/fs/ufs/super.c
> @@ -95,22 +95,18 @@
>  
>  void lock_ufs(struct super_block *sb)
>  {
> -#if defined(CONFIG_SMP) || defined (CONFIG_PREEMPT)
>  	struct ufs_sb_info *sbi = UFS_SB(sb);
>  
>  	mutex_lock(&sbi->mutex);
>  	sbi->mutex_owner = current;
> -#endif
>  }

Good heavens.

This patch is a bugfix.  lock_ufs() is assuming that on non-preempt
uniprocessor, the calling code will run atomically up to the matching
unlock_ufs().

But that isn't true.  The very first site I looked at (ufs_frag_map)
does sb_bread() under lock_ufs(.  And sb_bread() will call schedule(),
very commonly.

The ->mutex_owner stuff is a bit hacky but should work OK.

Care to resend the patch with a more alarming changelog?
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