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[3/4] scsi: libfc: Remove get_cpu() semantics in fc_exch_em_alloc()

Message ID 20220506105758.283887-4-bigeasy@linutronix.de (mailing list archive)
State Accepted
Headers show
Series scsi: PREEMPT_RT related fixes. | expand

Commit Message

Sebastian Andrzej Siewior May 6, 2022, 10:57 a.m. UTC 
From: Davidlohr Bueso <dave@stgolabs.net>

The get_cpu() in fc_exch_em_alloc() was introduced in:

    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.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20211117025956.79616-2-dave@stgolabs.net
---
 drivers/scsi/libfc/fc_exch.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
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Patch

diff --git a/drivers/scsi/libfc/fc_exch.c b/drivers/scsi/libfc/fc_exch.c
index aa223db4cf53c..1d91c457527f3 100644
--- a/drivers/scsi/libfc/fc_exch.c
+++ b/drivers/scsi/libfc/fc_exch.c
@@ -825,10 +825,9 @@  static struct fc_exch *fc_exch_em_alloc(struct fc_lport *lport,
 	}
 	memset(ep, 0, sizeof(*ep));
 
-	cpu = get_cpu();
+	cpu = raw_smp_processor_id();
 	pool = per_cpu_ptr(mp->pool, cpu);
 	spin_lock_bh(&pool->lock);
-	put_cpu();
 
 	/* peek cache of free slot */
 	if (pool->left != FC_XID_UNKNOWN) {