Subject: Re: [RESEND2 PATCH net v4 2/2] soc: fsl: qbman: Use raw spinlock for cgr_lock

From: Sean Anderson <[email-masked]>
Date: Fri, 23 Feb 2024 11:02:50 -0500

On 2/23/24 00:38, Christophe Leroy wrote:
> Le 22/02/2024 à 18:07, Sean Anderson a écrit :
>> [Vous ne recevez pas souvent de courriers de [email-masked]. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
>> 
>> cgr_lock may be locked with interrupts already disabled by
>> smp_call_function_single. As such, we must use a raw spinlock to avoid
>> problems on PREEMPT_RT kernels. Although this bug has existed for a
>> while, it was not apparent until commit ef2a8d5478b9 ("net: dpaa: Adjust
>> queue depth on rate change") which invokes smp_call_function_single via
>> qman_update_cgr_safe every time a link goes up or down.
> 
> Why a raw spinlock to avoid problems on PREEMPT_RT, can you elaborate ?

smp_call_function always runs its callback in hard IRQ context, even on
PREEMPT_RT, where spinlocks can sleep. So we need to use raw spinlocks
to ensure we aren't waiting on a sleeping task. See the first bug report
for more discussion.

In the longer term it would be better to switch to some other
abstraction.

--Sean

> If the problem is that interrupts are already disabled, shouldn't you 
> just change the spin_lock_irq() by spin_lock_irqsave() ?
>
> Christophe
> 
> 
>> 
>> Fixes: 96f413f47677 ("soc/fsl/qbman: fix issue in qman_delete_cgr_safe()")
>> CC: [email-masked]
>> Reported-by: Vladimir Oltean <[email-masked]>
>> Closes: https://lore.kernel.org/all/20230323153935.nofnjucqjqnz34ej@skbuf/
>> Reported-by: Steffen Trumtrar <[email-masked]>
>> Closes: https://lore.kernel.org/linux-arm-kernel/[email-masked]/
>> Signed-off-by: Sean Anderson <[email-masked]>
>> Reviewed-by: Camelia Groza <[email-masked]>
>> Tested-by: Vladimir Oltean <[email-masked]>
>> 
>> ---
>> 
>> Changes in v4:
>> - Add a note about how raw spinlocks aren't quite right
>> 
>> Changes in v3:
>> - Change blamed commit to something more appropriate
>> 
>>   drivers/soc/fsl/qbman/qman.c | 25 ++++++++++++++-----------
>>   1 file changed, 14 insertions(+), 11 deletions(-)
>> 
>> diff --git a/drivers/soc/fsl/qbman/qman.c b/drivers/soc/fsl/qbman/qman.c
>> index 1bf1f1ea67f0..7e9074519ad2 100644
>> --- a/drivers/soc/fsl/qbman/qman.c
>> +++ b/drivers/soc/fsl/qbman/qman.c
>> @@ -991,7 +991,7 @@ struct qman_portal {
>>          /* linked-list of CSCN handlers. */
>>          struct list_head cgr_cbs;
>>          /* list lock */
>> -       spinlock_t cgr_lock;
>> +       raw_spinlock_t cgr_lock;
>>          struct work_struct congestion_work;
>>          struct work_struct mr_work;
>>          char irqname[MAX_IRQNAME];
>> @@ -1281,7 +1281,7 @@ static int qman_create_portal(struct qman_portal *portal,
>>                  /* if the given mask is NULL, assume all CGRs can be seen */
>>                  qman_cgrs_fill(&portal->cgrs[0]);
>>          INIT_LIST_HEAD(&portal->cgr_cbs);
>> -       spin_lock_init(&portal->cgr_lock);
>> +       raw_spin_lock_init(&portal->cgr_lock);
>>          INIT_WORK(&portal->congestion_work, qm_congestion_task);
>>          INIT_WORK(&portal->mr_work, qm_mr_process_task);
>>          portal->bits = 0;
>> @@ -1456,11 +1456,14 @@ static void qm_congestion_task(struct work_struct *work)
>>          union qm_mc_result *mcr;
>>          struct qman_cgr *cgr;
>> 
>> -       spin_lock_irq(&p->cgr_lock);
>> +       /*
>> +        * FIXME: QM_MCR_TIMEOUT is 10ms, which is too long for a raw spinlock!
>> +        */
>> +       raw_spin_lock_irq(&p->cgr_lock);
>>          qm_mc_start(&p->p);
>>          qm_mc_commit(&p->p, QM_MCC_VERB_QUERYCONGESTION);
>>          if (!qm_mc_result_timeout(&p->p, &mcr)) {
>> -               spin_unlock_irq(&p->cgr_lock);
>> +               raw_spin_unlock_irq(&p->cgr_lock);
>>                  dev_crit(p->config->dev, "QUERYCONGESTION timeout\n");
>>                  qman_p_irqsource_add(p, QM_PIRQ_CSCI);
>>                  return;
>> @@ -1476,7 +1479,7 @@ static void qm_congestion_task(struct work_struct *work)
>>          list_for_each_entry(cgr, &p->cgr_cbs, node)
>>                  if (cgr->cb && qman_cgrs_get(&c, cgr->cgrid))
>>                          cgr->cb(p, cgr, qman_cgrs_get(&rr, cgr->cgrid));
>> -       spin_unlock_irq(&p->cgr_lock);
>> +       raw_spin_unlock_irq(&p->cgr_lock);
>>          qman_p_irqsource_add(p, QM_PIRQ_CSCI);
>>   }
>> 
>> @@ -2440,7 +2443,7 @@ int qman_create_cgr(struct qman_cgr *cgr, u32 flags,
>>          preempt_enable();
>> 
>>          cgr->chan = p->config->channel;
>> -       spin_lock_irq(&p->cgr_lock);
>> +       raw_spin_lock_irq(&p->cgr_lock);
>> 
>>          if (opts) {
>>                  struct qm_mcc_initcgr local_opts = *opts;
>> @@ -2477,7 +2480,7 @@ int qman_create_cgr(struct qman_cgr *cgr, u32 flags,
>>              qman_cgrs_get(&p->cgrs[1], cgr->cgrid))
>>                  cgr->cb(p, cgr, 1);
>>   out:
>> -       spin_unlock_irq(&p->cgr_lock);
>> +       raw_spin_unlock_irq(&p->cgr_lock);
>>          put_affine_portal();
>>          return ret;
>>   }
>> @@ -2512,7 +2515,7 @@ int qman_delete_cgr(struct qman_cgr *cgr)
>>                  return -EINVAL;
>> 
>>          memset(&local_opts, 0, sizeof(struct qm_mcc_initcgr));
>> -       spin_lock_irqsave(&p->cgr_lock, irqflags);
>> +       raw_spin_lock_irqsave(&p->cgr_lock, irqflags);
>>          list_del(&cgr->node);
>>          /*
>>           * If there are no other CGR objects for this CGRID in the list,
>> @@ -2537,7 +2540,7 @@ int qman_delete_cgr(struct qman_cgr *cgr)
>>                  /* add back to the list */
>>                  list_add(&cgr->node, &p->cgr_cbs);
>>   release_lock:
>> -       spin_unlock_irqrestore(&p->cgr_lock, irqflags);
>> +       raw_spin_unlock_irqrestore(&p->cgr_lock, irqflags);
>>          put_affine_portal();
>>          return ret;
>>   }
>> @@ -2577,9 +2580,9 @@ static int qman_update_cgr(struct qman_cgr *cgr, struct qm_mcc_initcgr *opts)
>>          if (!p)
>>                  return -EINVAL;
>> 
>> -       spin_lock_irqsave(&p->cgr_lock, irqflags);
>> +       raw_spin_lock_irqsave(&p->cgr_lock, irqflags);
>>          ret = qm_modify_cgr(cgr, 0, opts);
>> -       spin_unlock_irqrestore(&p->cgr_lock, irqflags);
>> +       raw_spin_unlock_irqrestore(&p->cgr_lock, irqflags);
>>          put_affine_portal();
>>          return ret;
>>   }
>> --
>> 2.35.1.1320.gc452695387.dirty
>>