linux-kernel - Re: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp

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Message-ID: <20141223074041.GB9946@jaegeuk-mac02.hsd1.ca.comcast.net>
Date:	Mon, 22 Dec 2014 23:40:41 -0800
From:	Jaegeuk Kim <jaegeuk@...nel.org>
To:	Chao Yu <chao2.yu@...sung.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp_rwsem

Hi,

On Tue, Dec 23, 2014 at 03:01:36PM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> > -----Original Message-----
> > From: Jaegeuk Kim [mailto:jaegeuk@...nel.org]
> > Sent: Monday, December 22, 2014 12:35 AM
> > To: linux-kernel@...r.kernel.org; linux-fsdevel@...r.kernel.org;
> > linux-f2fs-devel@...ts.sourceforge.net
> > Cc: Jaegeuk Kim
> > Subject: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp_rwsem
> > 
> > The __f2fs_add_link is covered by cp_rwsem all the time.
> > This calls init_inode_metadata, which conducts some acl operations including
> > memory allocation with GFP_KERNEL previously.
> > But, under memory pressure, f2fs_write_data_page can be called, which also
> > grabs cp_mutex too.
> 
> grabs cp_rwsem.

Got it.

> 
> > Basically, it's safe since down_read was used in both of cases, but it'd
> > better avoid this situation in advance.
> 
> If checkpoint intend to hold down_write in the middle of the two down_read
> caller, it will cause a deadlock, so it's not safe.

What I meant was like this.
- down_read
  - down_read
  - up_read
- up_read

This should be safe, right?
Thanks,

> 
> Could you update the comments?
> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> 
> Anyway, Nice catch and please add:
> 
> Reviewed-by: Chao Yu <chao2.yu@...sung.com>
> 
> > ---
> >  fs/f2fs/acl.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c
> > index 1ccb26b..7f12d28 100644
> > --- a/fs/f2fs/acl.c
> > +++ b/fs/f2fs/acl.c
> > @@ -62,7 +62,7 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
> >  	if (count == 0)
> >  		return NULL;
> > 
> > -	acl = posix_acl_alloc(count, GFP_KERNEL);
> > +	acl = posix_acl_alloc(count, GFP_NOFS);
> >  	if (!acl)
> >  		return ERR_PTR(-ENOMEM);
> > 
> > @@ -116,7 +116,7 @@ static void *f2fs_acl_to_disk(const struct posix_acl *acl, size_t *size)
> >  	int i;
> > 
> >  	f2fs_acl = kmalloc(sizeof(struct f2fs_acl_header) + acl->a_count *
> > -			sizeof(struct f2fs_acl_entry), GFP_KERNEL);
> > +			sizeof(struct f2fs_acl_entry), GFP_NOFS);
> >  	if (!f2fs_acl)
> >  		return ERR_PTR(-ENOMEM);
> > 
> > --
> > 2.1.1
> > 
> > 
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> > Linux-f2fs-devel@...ts.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
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