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

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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



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