Re: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp_rwsem

[Jaegeuk Kim]

On Tue, Dec 23, 2014 at 04:58:46PM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> > -----Original Message-----
> > From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> > Sent: Tuesday, December 23, 2014 3:41 PM
> > To: Chao Yu
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> > linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> > 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@xxxxxxxxxx]
> > > > Sent: Monday, December 22, 2014 12:35 AM
> > > > To: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> > > > linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> > > > 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
> 
> If someone else intends to hold down_write (in checkpoint()) here,
> we will encounter deadlock.
> 
> Because this writer will add itself to inner waiter list of spinlock as there
> is an exist reader had held this lock, then the following reader will start to
> spin as it detect that waiter list is not empty. Then deadlock occurred.

Ah ha. I see. :)
I'll change the description.

Thanks,

> 
> I guess this design can effectively avoid starve for the writer when encounters
> a large number of readers.
> 
> Thanks
> 
> >   - down_read
> >   - up_read
> > - up_read
> > 
> > This should be safe, right?
> > Thanks,
> > 
> > >
> > > Could you update the comments?
> > >
> > > >
> > > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > >
> > > Anyway, Nice catch and please add:
> > >
> > > Reviewed-by: Chao Yu <chao2.yu@xxxxxxxxxxx>
> > >
> > > > ---
> > > >  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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