Re: [PATCH] fs: nfs: fix a possible sleep-in-atomic-context bug in _pnfs_grab_empty_layout()

[Jia-Ju Bai]

On 2019/12/17 22:37, Trond Myklebust wrote:
> On Tue, 2019-12-17 at 21:33 +0800, Jia-Ju Bai wrote:
> > The filesystem may sleep while holding a spinlock.
> > The function call path (from bottom to top) in Linux 4.19 is:
> >
> > fs/nfs/pnfs.c, 2052:
> > 	pnfs_find_alloc_layout(GFP_KERNEL) in _pnfs_grab_empty_layout
> > fs/nfs/pnfs.c, 2051:
> > 	spin_lock in _pnfs_grab_empty_layout
> >
> > pnfs_find_alloc_layout(GFP_KERNEL) can sleep at runtime.
> >
> > To fix this possible bug, GFP_KERNEL is replaced with GFP_ATOMIC for
> > pnfs_find_alloc_layout().
> >
> > This bug is found by a static analysis tool STCheck written by
> > myself.
> >
> > Signed-off-by: Jia-Ju Bai <baijiaju1990@xxxxxxxxx>
> > ---
> >   fs/nfs/pnfs.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
> > index cec3070ab577..cfbe170f0651 100644
> > --- a/fs/nfs/pnfs.c
> > +++ b/fs/nfs/pnfs.c
> > @@ -2138,7 +2138,7 @@ _pnfs_grab_empty_layout(struct inode *ino,
> > struct nfs_open_context *ctx)
> >   	struct pnfs_layout_hdr *lo;
> >   
> >   	spin_lock(&ino->i_lock);
> > -	lo = pnfs_find_alloc_layout(ino, ctx, GFP_KERNEL);
> > +	lo = pnfs_find_alloc_layout(ino, ctx, GFP_ATOMIC);
> >   	if (!lo)
> >   		goto out_unlock;
> >   	if (!test_bit(NFS_LAYOUT_INVALID_STID, &lo->plh_flags))
> I'm not seeing why this is necessary. As far as I can see,
> pnfs_find_alloc_layout() will release the ino->i_lock before sleeping.
>
> False positive?

Thanks for the reply.
You are right, my report is false...
I did not check the definition of pnfs_find_alloc_layout(), sorry...


Best wishes,
Jia-Ju Bai