[PATCH v2] scsi: sg: Avoid race in error handling & drop bogus warn

[Alexander Wetzel]

commit 27f58c04a8f4 ("scsi: sg: Avoid sg device teardown race")
introduced an incorrect WARN_ON_ONCE() and missed a sequence where
sg_device_destroy() was used after scsi_device_put().

sg_device_destroy() is accessing the parent scsi_device request_queue which
will already be set to NULL when the preceding call to scsi_device_put()
removed the last reference to the parent scsi_device.

Drop the incorrect WARN_ON_ONCE() - allowing more than one concurrent
access to the sg device - and make sure sg_device_destroy() is not used
after scsi_device_put() in the error handling.

Link: https://lore.kernel.org/all/5375B275-D137-4D5F-BE25-6AF8ACAE41EF@linux.ibm.com
Fixes: 27f58c04a8f4 ("scsi: sg: Avoid sg device teardown race")
Cc: stable at vger.kernel.org
Signed-off-by: Alexander Wetzel <Alexander at wetzel-home.de>
---

Changes compared to V1: fixed commit message

The WARN_ON_ONCE() was kind of stupid to add:
We get add reference for each sg_open(). So opening a second session and
then closing either one will trigger the warning... Nothing to warn
about here.

Alexander
---
 drivers/scsi/sg.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 386981c6976a..833c9277419b 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -372,8 +372,9 @@ sg_open(struct inode *inode, struct file *filp)
 error_out:
 	scsi_autopm_put_device(sdp->device);
 sdp_put:
+	kref_put(&sdp->d_ref, sg_device_destroy);
 	scsi_device_put(sdp->device);
-	goto sg_put;
+	return retval;
 }
 
 /* Release resources associated with a successful sg_open()
@@ -2233,7 +2234,6 @@ sg_remove_sfp_usercontext(struct work_struct *work)
 			"sg_remove_sfp: sfp=0x%p\n", sfp));
 	kfree(sfp);
 
-	WARN_ON_ONCE(kref_read(&sdp->d_ref) != 1);
 	kref_put(&sdp->d_ref, sg_device_destroy);
 	scsi_device_put(device);
 	module_put(THIS_MODULE);
-- 
2.44.0