linux-kernel - [PATCH 1/3] scsi/libfc: Remove get_cpu() semantics in fc_exch_em

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Date:   Tue, 16 Nov 2021 18:59:54 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     martin.petersen@...cle.com, jejb@...ux.ibm.com
Cc:     hare@...e.de, bigeasy@...utronix.de, tglx@...utronix.de,
        linux-scsi@...r.kernel.org, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, dave@...olabs.net,
        Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 1/3] scsi/libfc: Remove get_cpu() semantics in fc_exch_em_alloc()

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@...e.de>
---
 drivers/scsi/libfc/fc_exch.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/scsi/libfc/fc_exch.c b/drivers/scsi/libfc/fc_exch.c
index 841000445b9a..be37bfb2814d 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) {
-- 
2.26.2