[PATCH] NFSv4: clear exception state on successful mkdir retry

[Igor Raits <igor.raits@gmail.com>]

After a server returns NFS4ERR_DELAY for an NFSv4 CREATE issued by
mkdir(2), the client correctly waits and retries.  When the retry
succeeds, however, mkdir(2) can still surface -EEXIST to userspace
even though the directory was just created on the server.

Reproducer (random 16-hex names so collisions are not the cause)
against an in-kernel Linux nfsd; reproduces under both NFSv4.0 and
NFSv4.2:

  N=2000000; base=/var/gdc/export
  for ((i=1; i<=N; i++)); do
      d=$base/$(openssl rand -hex 8)
      mkdir "$d" 2>/dev/null || echo "$(date +%T) failed loop=$i $d"
      rmdir "$d" 2>/dev/null
  done

Failures cluster at the cadence at which the server-side auth/export
cache refresh path causes nfsd to return NFS4ERR_DELAY for CREATE.

A wire trace of one failure (the three CREATE RPCs all come from a
single mkdir(2), generated by the do-while in nfs4_proc_mkdir()):

  client -> server  CREATE name=...  -> NFS4ERR_DELAY
  ~100 ms later
  client -> server  CREATE name=...  -> NFS4_OK         (dir created)
  ~80 us later
  client -> server  CREATE name=...  -> NFS4ERR_EXIST   (correct)

Since commit dd862da61e91 ("nfs: fix incorrect handling of large-number
NFS errors in nfs4_do_mkdir()"), nfs4_handle_exception() is called only
when _nfs4_proc_mkdir() returned an error.  That gate breaks retry-state
hygiene: nfs4_do_handle_exception() resets exception.{delay,recovering,
retry} to 0 on entry, so calling it on success is what previously
cleared the retry flag set by the preceding NFS4ERR_DELAY iteration.
With the gate in place, exception.retry stays at 1 after the successful
retry, the loop runs once more, and the resulting CREATE for an
already-created name yields NFS4ERR_EXIST -> -EEXIST to userspace.

Drop the conditional and call nfs4_handle_exception() unconditionally,
matching every other do-while in fs/nfs/nfs4proc.c (nfs4_proc_symlink(),
nfs4_proc_link(), etc.).  The dentry/status separation introduced by
that commit is preserved.

Fixes: dd862da61e91 ("nfs: fix incorrect handling of large-number NFS errors in nfs4_do_mkdir()")
Reported-and-tested-by: Jan Čípa <jan.cipa@gooddata.com>
Closes: https://lore.kernel.org/linux-nfs/CA+9S74hSp_tJu2Ffe2BPNC2T25gfkhgjjDkdgSsF5c2rnJq_wA@mail.gmail.com/
Reviewed-by: NeilBrown <neil@brown.name>
Cc: stable@vger.kernel.org
Signed-off-by: Igor Raits <igor.raits@gmail.com>
---
 fs/nfs/nfs4proc.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index a0885ae55abc..ffd14141ea1d 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -5393,10 +5393,9 @@ static struct dentry *nfs4_proc_mkdir(struct inode *dir, struct dentry *dentry,
 	do {
 		alias = _nfs4_proc_mkdir(dir, dentry, sattr, label, &err);
 		trace_nfs4_mkdir(dir, &dentry->d_name, err);
+		err = nfs4_handle_exception(NFS_SERVER(dir), err, &exception);
 		if (err)
-			alias = ERR_PTR(nfs4_handle_exception(NFS_SERVER(dir),
-							      err,
-							      &exception));
+			alias = ERR_PTR(err);
 	} while (exception.retry);
 	nfs4_label_release_security(label);
 
-- 
2.53.0