Allowing a non-power-of-2 zone size forces the use of direct division
operations of 64-bit sector values to obtain a zone number or number of
zones. Doing so without using do_div() leads to compilation errors on
32-bit architectures.

Devices with a zone size that is not a power of 2 do not exist today so
allowing their emulation is of limited interest as the sd driver will not
support them anyway. To fix this compilation error, instead of using
do_div() for sector values divisions, simply disallow zone size values that
are not a power of 2.

[mkp: commit desc]

Link: https://lore.kernel.org/r/20200507023526.221574-1-damien.lemoal@wdc.com
Fixes: 98e0a689 ("scsi: scsi_debug: Add zone_size_mb module parameter")
Fixes: f0d1cf93 ("scsi: scsi_debug: Add ZBC zone commands")
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Implement ZBC host-aware device model emulation. The main changes from the
host-managed emulation are the device type (TYPE_DISK is used), relaxation
of access checks for read and write operations and different handling of a
sequential write preferred zone write pointer as mandated by the ZBC r05
specifications.

To facilitate the implementation and avoid a lot of "if" statement, the
zmodel field is added to the device information and the z_type field to the
zone state data structure.

Link: https://lore.kernel.org/r/20200422104221.378203-8-damien.lemoal@wdc.comTested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add the zone_size_mb module parameters to control the zone size of a ZBC
device. If the zone size specified is not a divisor of the device capacity,
the last zone of the device will be created as a smaller "runt" zone. This
parameter is ignored for device types other than 0x14 (zbc=2 case).

Note: for testing purposes, zone sizes that are not a power of 2 are
accepted but will result in the drive being rejected by the sd driver.

Link: https://lore.kernel.org/r/20200422104221.378203-7-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Allow controlling the number of conventional zones of a ZBC device with the
new zone_nr_conv module parameter. The default value is 1 and the specified
value must be less than the total number of zones of the device. This
parameter is ignored for device types other than 0x14 (zbc=2 case).

Link: https://lore.kernel.org/r/20200422104221.378203-6-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add the zone_max_open module parameters to control the maximum number of
open zones of a ZBC device. This parameter is ignored for device types
other than 0x14 (zbc=2 case).

Link: https://lore.kernel.org/r/20200422104221.378203-5-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add the zbc module parameter to take either:
    0: none         (probably a conventional disk)
    1: host-aware
    2: host-managed

These values are chosen to match 'enum blk_zoned_model' found in
include/linux/blkdev.h . Instead of "none", "no" or "0" can be given.
Instead of "host-aware", "aware or "1" can be given. Instead of
"host-managed", "managed" or "2" can be given.

Note: the zbc parameter can only be given at driver/module load time; it
cannot be changed via sysfs thereafter.

At this time there is no ZBC "host-aware" implementation so that string (or
the value '1') results in a modprobe error.

Link: https://lore.kernel.org/r/20200422104221.378203-4-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add support for the 5 ZBC commands and enough functionality to emulate a
host-managed device with one conventional zone and a set of sequential
write-required zones up to the disk capacity.

Link: https://lore.kernel.org/r/20200422104221.378203-3-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The ZBC standard "piggy-backs" on many, but not all, of the facilities in
SBC. Add those ZBC mode pages (plus mode parameter block descriptors
(e.g. "WP")) and VPD pages in common with SBC. Add ZBC specific VPD page
for the host-managed ZBC device type (ptype=0x14).

Link: https://lore.kernel.org/r/20200422104221.378203-2-damien.lemoal@wdc.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The scsi_debug driver version is visible in:

   /sys/modules/scsi_debug/version

and can thus be used by user space programs to alter the features they try
to use. Since the per_host_store and zbc/zone options are significant
additions, bump the version number to 1.89 .

Link: https://lore.kernel.org/r/20200421151424.32668-9-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

This module has a lot of parameters and when searching for one, the author
prefers them in alphabetical order. This can lead to somewhat illogical
ordering (e.g. inq_product before inq_vendor). However it is not clear what
another sensible total logical ordering would be.

Link: https://lore.kernel.org/r/20200421151424.32668-8-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Many disks implement the SCSI PRE-FETCH commands. One use case might be a
disk-to-disk compare, say between disks A and B. Then this sequence of
commands might be used: PRE-FETCH(from B, IMMED), READ(from A), VERIFY
(BYTCHK=1 on B with data returned from READ).  The PRE-FETCH (which returns
quickly due to the IMMED) fetches the data from the media into B's cache
which should speed the trailing VERIFY command. The next chunk of the
compare might be done in parallel, with A and B reversed.

The implementation tries to bring the specified range in main memory into
the cache(s) associated with this machine's CPU(s) using the
prefetch_range() function.

Link: https://lore.kernel.org/r/20200421151424.32668-7-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Previously the code did the work implied by the given SCSI command and
after that it waited for a timer based on the user specified command
duration to be exhausted before informing the mid-level that the command
was complete. For short command durations, the time to complete the work
implied by the SCSI command could be significant compared to the user
specified command duration.

For example a WRITE of 128 blocks (say 512 bytes each) on a machine that
can copy from main memory to main memory at a rate of 10 GB/sec will take
around 6.4 microseconds to do that copy.  If the user specified a command
duration of 5 microseconds (ndelay=5000), should the driver do a further
delay of 5 microseconds after the copy or return immediately because 6.4 >
5 ?

The action prior to this patch was to always do the timer based
delay. After this patch, for ndelay values less than 1 millisecond, this
driver will complete the command immediately.  And in the case where the
user specified delay was 7 microseconds, a timer delay of 600 nanoseconds
will be set ((7 - 6.4) * 1000).

Link: https://lore.kernel.org/r/20200421151424.32668-6-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The design of this driver is to do any ramdisk access on the same thread
that invoked the queuecommand() call. That is assumed to be user space
context. The command duration is implemented by setting the delay with a
high resolution timer. The hr timer's callback may well be in interrupt
context, but it doesn't touch the ramdisk. So try removing the
_irqsave()/_irqrestore() portion on the read-write lock that protects
ramdisk access.

Link: https://lore.kernel.org/r/20200421151424.32668-5-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

With the addition of the per_host_store option, the ability to check
whether two different ramdisk images are the same or not becomes
practical. Prior to this patch VERIFY(10) always returned true (i.e.  the
SCSI GOOD status) without checking. This option adds support for BYTCHK
equal to 0, 1 and 3. If the comparison fails, then a sense key of
MISCOMPARE is returned as per the T10 standards. Also add support for the
VERIFY(16) command.

Link: https://lore.kernel.org/r/20200421151424.32668-4-dgilbert@interlog.comReviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The scsi_debug driver has always been restricted to using one ramdisk image
(or none) for its storage. This means that thousands of scsi_debug devices
can be created without exhausting the host machine's RAM. The downside is
that all scsi_debug devices share the same ramdisk image. This option
changes the way a following write to the add_host parameter (or an add_host
in the module/driver invocation) operates.  For each new host that is
created while per_host_store is true, a new store (of dev-size_mb MiB) is
created and associated with all the LUs that belong to that new host. The
user (who will need root permissions) needs to take care not to exhaust all
the machine's available RAM.

One reason for doing this is to check that (partial) disk to disk copies
based on scsi_debug devices have actually copied accurately. To test this
the add_host=<n> parameter where <n> is 2 or greater can be used when the
scsi_debug module is loaded. Let us assume that /dev/sdb and /dev/sg1 are
the same scsi_debug device, while /dev/sdc and /dev/sg2 are the same
scsi_debug device. With per_host_store=1 add_host=2 they will have
different ramdisk images. Then the following pseudocode could be executed
to check if the sgh_dd copy worked:

    dd if=/dev/urandom of=/dev/sdb
    sgh_dd if=/dev/sg1 of=/dev/sg2 [plus option(s) to test]
    cmp /dev/sdb /dev/sdc

If the cmp fails then the copy has failed (or some other mechanism wrote to
/dev/sdb or /dev/sdc in the interim).

[mkp: use kstrtobool()]

Link: https://lore.kernel.org/r/20200421151424.32668-3-dgilbert@interlog.comSigned-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add a new command line option (e.g. random=1) and sysfs attribute that
causes subsequent command completion times to be between the current
command delay setting and 0. A uniformly distributed 32 bit, kernel
provided integer is used for this purpose.

Since the existing 'delay' whose units are jiffies (typically milliseconds)
and 'ndelay' (units: nanoseconds) options (and sysfs attributes) span a
range greater than 32 bits, some scaling is required.

The purpose of this patch is to widen the range of testing cases that are
visited in long running tests. Put simply: rarely struct race conditions
are more likely to be found when this facility is used.

The default is the previous case in which all command completions were
roughly equal to (if not, slightly longer) than the value given by the
'delay' or 'ndelay' settings (or their defaults).  This option's default is
equivalent to setting 'random=0' .

[mkp: use kstrtobool()]

Link: https://lore.kernel.org/r/20200421151424.32668-2-dgilbert@interlog.comReviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The channel index is represented by an unsigned variable 'u32 chan'. We
don't need to check whether it is less than zero, the 'chan < 0' statement
is always false. Remove it.

Link: https://lore.kernel.org/r/1588162218-61757-1-git-send-email-wangxiongfeng2@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Smatch complains that the "path_data->handle" variable is user controlled.
It comes from iscsi_set_path() so that seems possible.  It's harmless to
add a limit check.

The qedi->ep_tbl[] array has qedi->max_active_conns elements (which is
always ISCSI_MAX_SESS_PER_HBA (4096) elements).  The array is allocated in
the qedi_cm_alloc_mem() function.

Link: https://lore.kernel.org/r/20200428131939.GA696531@mwanda
Fixes: ace7f46b ("scsi: qedi: Add QLogic FastLinQ offload iSCSI driver framework.")
Acked-by: Manish Rangankar <mrangankar@marvell.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The bitfields mpi_fw_dump_reading and mpi_fw_dumped are currently signed
which is not recommended as the representation is an implementation defined
behaviour.  Fix this by making the bit-fields unsigned ints.

Link: https://lore.kernel.org/r/20200428102013.1040598-1-colin.king@canonical.com
Fixes: cbb01c2f ("scsi: qla2xxx: Fix MPI failure AEN (8200) handling")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

In case scsi_setup_fs_cmnd() fails we're not freeing the sgtables allocated
by scsi_init_io(), thus we leak the allocated memory.

Free the sgtables allocated by scsi_init_io() in case scsi_setup_fs_cmnd()
fails.

Technically scsi_setup_scsi_cmnd() does not suffer from this problem as it
can only fail if scsi_init_io() fails, so it does not have sgtables
allocated. But to maintain symmetry and as a measure of defensive
programming, free the sgtables on scsi_setup_scsi_cmnd() failure as well.
scsi_mq_free_sgtables() has safeguards against double-freeing of memory so
this is safe to do.

While we're at it, rename scsi_mq_free_sgtables() to scsi_free_sgtables().

Link: https://bugzilla.kernel.org/show_bug.cgi?id=205595
Link: https://lore.kernel.org/r/20200428104605.8143-2-johannes.thumshirn@wdc.comReviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Despite of functions being documented, they are not in the kernel-doc
specification, and could not be included in kernel documentation. Change
the style of functions comments to be compliant to the kernel-doc style.
When the function comments are outdated, update then.

[mkp: a few edits]

Link: https://lore.kernel.org/r/20200419050148.33371-1-andrealmeid@collabora.comSigned-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Enable WriteBooster for Qualcomm platform.

Link: https://lore.kernel.org/r/cd4cf745ea0b3a59c2075036e17316b97494fe65.1587591527.git.asutoshd@codeaurora.orgReviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Adds unit, device, geometry descriptor sysfs entries.  Adds flags sysfs
entries for write booster.

Link: https://lore.kernel.org/r/98987ef17844292bd42c57613990a3a26c6de2b8.1587591527.git.asutoshd@codeaurora.orgReviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

The write performance of TLC NAND is considerably lower than SLC NAND.
Using SLC NAND as a WriteBooster Buffer enables the write request to be
processed with lower latency and improves the overall write performance.

Adds support for shared-buffer mode WriteBooster.

WriteBooster enable: SW enables it when clocks are scaled up, thus it's
enabled only in high load conditions.

WriteBooster disable: SW will disable the feature, when clocks are scaled
down. Thus writes would go as normal writes.

To keep the endurance of the WriteBooster Buffer at a maximum, this
load-based toggling is adopted.

Link: https://lore.kernel.org/r/2871444d9083b0e9323ef6d8ff1b544b7784adc9.1587591527.git.asutoshd@codeaurora.orgReviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/ufs/ufshcd.c:4140:6-14: WARNING: Assignment of 0/1 to bool
variable.

Link: https://lore.kernel.org/r/20200426094305.24083-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

This patch makes the sr code slightly easier to read.

Link: https://lore.kernel.org/r/20200427014844.12109-1-bvanassche@acm.org
Cc: Merlijn Wajer <merlijn@archive.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

If 'scsi_host_alloc()' or 'kcalloc()' fail, 'error' is known to be 0. Set
it explicitly to -ENOMEM before branching to the error handling path.

While at it, remove 2 useless assignments to 'error'. These values are
overwridden a few lines later.

Link: https://lore.kernel.org/r/20200412094039.8822-1-christophe.jaillet@wanadoo.frSigned-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/sgiwd93.c:190:2-3: Unneeded semicolon

Link: https://lore.kernel.org/r/20200421034029.28030-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/qla4xxx/ql4_os.c:969:3-4: Unneeded semicolon

Link: https://lore.kernel.org/r/20200421034038.28113-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/isci/isci.h:515:1-12: WARNING: Assignment of 0/1 to bool
variable
drivers/scsi/isci/isci.h:503:1-12: WARNING: Assignment of 0/1 to bool
variable
drivers/scsi/isci/isci.h:509:1-12: WARNING: Assignment of 0/1 to bool
variable

Link: https://lore.kernel.org/r/20200421034050.28193-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/bnx2fc/bnx2fc_fcoe.c:948:4-5: Unneeded semicolon
drivers/scsi/bnx2fc/bnx2fc_fcoe.c:968:4-5: Unneeded semicolon

Link: https://lore.kernel.org/r/20200421034019.27949-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/bfa/bfa_fcs_rport.c:2452:2-3: Unneeded semicolon
drivers/scsi/bfa/bfa_fcs_rport.c:1578:3-4: Unneeded semicolon

Link: https://lore.kernel.org/r/20200421033957.27783-1-yanaijie@huawei.comSigned-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/scsi/bfa/bfa_svc.c: In function 'uf_recv':
drivers/scsi/bfa/bfa_svc.c:5520:17: warning:
 variable 'fchs' set but not used [-Wunused-but-set-variable]
  struct fchs_s *fchs;
                 ^

Link: https://lore.kernel.org/r/20200418071057.96699-1-yuehaibing@huawei.comSigned-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

This function does not need a return value since no callers depend on
it. Make it return void.

This also fixes the coccicheck warning:

drivers/scsi/snic/snic_ctl.c:163:5-8: Unneeded variable: "ret". Return
"0" on line 228

Link: https://lore.kernel.org/r/20200418070615.11603-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/mpt3sas/mpt3sas_base.c:4906:3-19: WARNING: NULL check
before some freeing functions is not needed.

Link: https://lore.kernel.org/r/20200418095850.34883-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/ipr.c:9533:2-18: WARNING: NULL check before some freeing
functions is not needed.

Link: https://lore.kernel.org/r/20200418095903.35118-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Fix the following coccicheck warning:

drivers/scsi/bfa/bfa_fcs_lport.c:4361:3-4: Unneeded semicolon

Link: https://lore.kernel.org/r/20200418070553.11262-1-yanaijie@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Replace dma_pool_malloc with dma_pool_zalloc to make the code more concise
in pmcraid_allocate_control_blocks() function.

Link: https://lore.kernel.org/r/1587197241-274646-1-git-send-email-wubo40@huawei.comSigned-off-by: Wu Bo <wubo40@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

This patch removes the iscsi_data_count structure and the
iscsit_do_rx_data() function because they are used only by rx_data()

Link: https://lore.kernel.org/r/20200424113913.17237-1-mlombard@redhat.comReviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

scsi_host_block() calls scsi_internal_device_block() for each scsi_device and
scsi_internal_device_block() calls blk_mq_quiesce_queue() for each LUN.

Since synchronize_rcu() is called from blk_mq_quiesce_queue(), this can cause
substantial slowdowns on systems with many LUNs.

Use scsi_internal_device_block_nowait() to implement scsi_host_block() so it
is sufficient to run synchronize_rcu() once. This is safe since SCSI does not
set the BLK_MQ_F_BLOCKING flag.

[mkp: commit desc and comment tweaks]

Link: https://lore.kernel.org/r/20200423020713.332743-1-ming.lei@redhat.com
Cc: Steffen Maier <maier@linux.ibm.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>