Messages in this thread | | | Date | Wed, 17 Nov 2021 08:25:41 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH 1/3] scsi/libfc: Remove get_cpu() semantics in fc_exch_em_alloc() |
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On 2021-11-16 18:59:54 [-0800], Davidlohr Bueso wrote: > 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>
> 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();
The `cpu' variable is later ORed into ep->oxid. I haven't figured out why this is important/ required. The ep variable/ `fc_exch' is allocated from a per-CPU memory pool and is later released to the same pool but the pool's CPU number is not obtained from fc_exch::oxid but there is a pointer to its pool stored in fc_exch::em. So it remains a mystery to why the CPU number is stored here.
> /* peek cache of free slot */ > if (pool->left != FC_XID_UNKNOWN) {
Sebastian
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