On Wed, Jan 31, 2024 at 11:17:40AM +1100, NeilBrown wrote:
>
> It is possible for free_blocked_lock() to be called twice concurrently,
> once from nfsd4_lock() and once from nfsd4_release_lockowner() calling
> remove_blocked_locks(). This is why a kref was added.
>
> It is perfectly safe for locks_delete_block() and kref_put() to be
> called in parallel as they use locking or atomicity respectively as
> protection. However locks_release_private() has no locking. It is
> safe for it to be called twice sequentially, but not concurrently.
>
> This patch moves that call from free_blocked_lock() where it could race
> with itself, to free_nbl() where it cannot. This will slightly delay
> the freeing of private info or release of the owner - but not by much.
> It is arguably more natural for this freeing to happen in free_nbl()
> where the structure itself is freed.
>
> This bug was found by code inspection - it has not been seen in practice.
>
> Fixes: 47446d74f170 ("nfsd4: add refcount for nfsd4_blocked_lock")
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> fs/nfsd/nfs4state.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index a66d66b9f769..12534e12dbb3 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -318,6 +318,7 @@ free_nbl(struct kref *kref)
> struct nfsd4_blocked_lock *nbl;
>
> nbl = container_of(kref, struct nfsd4_blocked_lock, nbl_kref);
> + locks_release_private(&nbl->nbl_lock);
> kfree(nbl);
> }
>
> @@ -325,7 +326,6 @@ static void
> free_blocked_lock(struct nfsd4_blocked_lock *nbl)
> {
> locks_delete_block(&nbl->nbl_lock);
> - locks_release_private(&nbl->nbl_lock);
> kref_put(&nbl->nbl_kref, free_nbl);
> }
>
> --
> 2.43.0
Applied to nfsd-next (for v6.9).
--
Chuck Lever
