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The memories for the slot should be observed to be written prior to
observing the slot as ready.

Prior to commit 26fc0ea7 ("scsi: libsas: Drop SAS_TASK_AT_INITIATOR"),
we had a spin_lock() + spin_unlock() immediately before marking the slot as
ready. The spin_unlock() - with release semantics - caused the slot memory
to be observed to be written.

Now that the spin_lock() + spin_unlock() is gone, use a smp_wmb().

Link: https://lore.kernel.org/r/1652774661-12935-1-git-send-email-john.garry@huawei.com
Fixes: 26fc0ea7

 ("scsi: libsas: Drop SAS_TASK_AT_INITIATOR")
Reported-by: Yihang Li <liyihang6@hisilicon.com>
Tested-by: Yihang Li <liyihang6@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>