[PATCH] pNFS: deadlock in pnfs_send_layoutreturn

[Ben Roberts]

Apologies, resending due to improper mail client settings on earlier
attempts.

On a HPC cluster running 5.14.0-611.9.1.el9.x86_64, regular deadlocks were seen
within pnfs_send_layoutreturn leading to userspace processes stuck in
uninterruptible sleep, ultimately requiring reboots to clear. This was occurring
frequently, sometimes multiple times per day on specific hosts with heavy load.
Claude code was tasked with hunting down any potential deadlocks within
pnfs_send_layoutreturn, and identified the following condition. This patch has
been running in production on top of the EL9 kernel for over three months
without any reoccurrence of the deadlock.

The pnfs_send_layoutreturn() function can deadlock when memory
allocation fails. The issue occurs in the error path where
pnfs_put_layout_hdr() is called, which may trigger
pnfs_layoutreturn_before_put_layout_hdr(), potentially causing
a recursive call back to pnfs_send_layoutreturn().

Call chain that triggers the deadlock:
1. pnfs_send_layoutreturn() - kzalloc() fails
2. Error path calls pnfs_put_layout_hdr(lo)
3. pnfs_put_layout_hdr() calls pnfs_layoutreturn_before_put_layout_hdr()
4. If NFS_LAYOUT_RETURN_REQUESTED is still set, attempts another
   layoutreturn, creating recursion/deadlock

The fix ensures that NFS_LAYOUT_RETURN_REQUESTED is cleared in the
allocation failure path before calling pnfs_put_layout_hdr(). This
prevents pnfs_layoutreturn_before_put_layout_hdr() from attempting
another layout return, breaking the recursion cycle.

Signed-off-by: Ben Roberts <ben.roberts@xxxxxxxxxxxxxx>
Assisted-by: Claude:claude-sonnet-4-5
---
 fs/nfs/pnfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index bc13d1e69449..47bda53b2b3a 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -1361,6 +1361,7 @@ pnfs_send_layoutreturn(struct pnfs_layout_hdr *lo,
 	if (unlikely(lrp == NULL)) {
 		status = -ENOMEM;
 		spin_lock(&ino->i_lock);
+		pnfs_clear_layoutreturn_info(lo)
 		pnfs_clear_layoutreturn_waitbit(lo);
 		spin_unlock(&ino->i_lock);
 		put_cred(cred);
--
2.43.0

For details of how GSA uses your personal information, please see our Privacy Notice here: https://www.gsacapital.com/privacy-notice 

This email and any files transmitted with it contain confidential and proprietary information and is solely for the use of the intended recipient.
If you are not the intended recipient please return the email to the sender and delete it from your computer and you must not use, disclose, distribute, copy, print or rely on this email or its contents.
This communication is for informational purposes only.
It is not intended as an offer or solicitation for the purchase or sale of any financial instrument or as an official confirmation of any transaction.
Any comments or statements made herein do not necessarily reflect those of GSA Capital.
GSA Capital Partners LLP is authorised and regulated by the Financial Conduct Authority and is registered in England and Wales at Stratton House, 5 Stratton Street, London W1J 8LA, number OC309261.
GSA Capital Services Limited is registered in England and Wales at the same address, number 5320529.