[PATCH 04/18] scsi_transport_sas: fix delete vs scan race

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The following crash results from cases where the end_device has been
removed before scsi_sysfs_add_sdev has had a chance to run.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
 IP: [<ffffffff8115e100>] sysfs_create_dir+0x32/0xb6
 ...
 Call Trace:
  [<ffffffff8125e4a8>] kobject_add_internal+0x120/0x1e3
  [<ffffffff81075149>] ? trace_hardirqs_on+0xd/0xf
  [<ffffffff8125e641>] kobject_add_varg+0x41/0x50
  [<ffffffff8125e70b>] kobject_add+0x64/0x66
  [<ffffffff8131122b>] device_add+0x12d/0x63a
  [<ffffffff814b65ea>] ? _raw_spin_unlock_irqrestore+0x47/0x56
  [<ffffffff8107de15>] ? module_refcount+0x89/0xa0
  [<ffffffff8132f348>] scsi_sysfs_add_sdev+0x4e/0x28a
  [<ffffffff8132dcbb>] do_scan_async+0x9c/0x145

...teach sas_rphy_remove to wait for async scanning to quiesce before
removing the end_device.  It seems this is a more general problem [1],
but this patch only addresses sas transport.

[1]: 23edb6e [SCSI] mpt2sas: Do not set sas_device->starget to NULL from
the slave_destroy callback when all the LUNS have been deleted

Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 drivers/scsi/scsi_transport_sas.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c
index f7565fc..47abb90 100644
--- a/drivers/scsi/scsi_transport_sas.c
+++ b/drivers/scsi/scsi_transport_sas.c
@@ -33,8 +33,9 @@
 #include <linux/bsg.h>
 
 #include <scsi/scsi.h>
-#include <scsi/scsi_device.h>
 #include <scsi/scsi_host.h>
+#include <scsi/scsi_scan.h>
+#include <scsi/scsi_device.h>
 #include <scsi/scsi_transport.h>
 #include <scsi/scsi_transport_sas.h>
 
@@ -1667,6 +1668,9 @@ sas_rphy_remove(struct sas_rphy *rphy)
 {
 	struct device *dev = &rphy->dev;
 
+	/* prevent device_del() while child device_add() may be in-flight */
+	scsi_complete_async_scans();
+
 	switch (rphy->identify.device_type) {
 	case SAS_END_DEVICE:
 		scsi_remove_target(dev);

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