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scsi: libfc: Remove get_cpu() semantics in fc_exch_em_alloc()

Luis Claudio R. Goncalves requested to merge lgoncalv/centos-stream-9:9.x__qedf_fcoe into main

JIRA: https://issues.redhat.com/browse/RHEL-2169

commit a0548edf852a8776dade1e511694b2980c674e2a
Author: Davidlohr Bueso dave@stgolabs.net
Date: Fri May 6 12:57:57 2022 +0200

scsi: libfc: Remove get_cpu() semantics in fc_exch_em_alloc()  

The get_cpu() in fc_exch_em_alloc() was introduced in commit f018b73af6db  
("[SCSI] libfc, libfcoe, fcoe: use smp_processor_id() only when preempt  
disabled") for no other reason than to simply use smp_processor_id()  
without getting a warning, because everything is done with the pool->lock  
held anyway.  However, get_cpu(), by disabling preemption, does not play  
well with PREEMPT_RT, particularly when acquiring a regular (and thus  
sleepable) spinlock.  

Therefore remove the get_cpu() and just use the unstable value as we will  
have CPU locality guarantees next by taking the lock.  The window of  
migration, as noted by Sebastian, is small and even if it happens the  
result is correct.  

Link: https://lore.kernel.org/r/20211117025956.79616-2-dave@stgolabs.net  
Link: https://lore.kernel.org/r/20220506105758.283887-4-bigeasy@linutronix.de  
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>  
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>  
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>  
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Signed-off-by: Luis Claudio R. Goncalves lgoncalv@redhat.com

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