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SubjectRe: Prevent busy looping
Tejun Heo <> wrote:
> Tejun Heo wrote:
>> Alan Cox wrote:
>>>> Elias's synthetic test case triggered infinite loop because it wasn't
>>>> a proper ->qc_defer(). ->qc_defer() should never defer commands when
>>>> the target is idle.
>>> Target or host ? We *do* defer commands in the case of an idle channel
>>> when dealing with certain simplex controllers that can only issue one
>>> command per host not one per cable (and in fact in the general case we
>>> can defer commands due to activity on the other drive on the cable).
>> The term was confusing. I used target to mean both device
>> (ATA_DEFER_LINK) and host (ATA_DEFER_PORT). Hmmm... in simplex case,
>> yeah, blocked counters need to be > 1. We'll need to increase blocked
>> counts after all. I'll test blocked counts of 2 w/ PMP and make sure it
>> doesn't incur unnecessary delays and post the patch.
> Setting blocked counts to 2 makes simplex scheduling starve one of the
> drives. When a drive loses competition, it retries only after plug
> delay and of course it loses most of the time. For now, it seems we'll
> have to live with busy loops (which doesn't lock up the machine) for
> simplex controllers. Ewww... :-(

Since I'm a little confused by your comment, please explain again. Do
you mean to say that busy looping doesn't lock up the machine in general
or merely in the case of a simplex configuration?

The reason why I'm asking is this: The whole point of my synthetic
->qc_defer() function was to prove that command deferral could (under
certain conditions) lead to busy looping which *did* lock up my machine.
Lock up in this context means that there was no response whatsoever to
key presses and even timers didn't fire anymore. I can see your point
that my ->qc_defer() function doesn't reflect reality very well because
the device is idle at the time and therefore no interrupts can be
expected from there. However, I still think that interrupts won't even
be processed once busy looping has started (in some configurations at

You can find a slightly modified version of my synthetic ->qc_defer()
function below (apply to 2.6.26-rc5) which demonstrates that at least
soft interrupts don't get serviced anymore once the busy looping has
started. Considering this, how can I be sure that an interrupt of the
target would be processed, even if it was not idle?



drivers/ata/ata_piix.c | 37 +++++++++++++++++++++++++++++++++++++
1 files changed, 37 insertions(+), 0 deletions(-)

diff --git a/drivers/ata/ata_piix.c b/drivers/ata/ata_piix.c
index 81b7ae3..9816daa 100644
--- a/drivers/ata/ata_piix.c
+++ b/drivers/ata/ata_piix.c
@@ -167,6 +167,7 @@ static int ich_pata_cable_detect(struct ata_port *ap);
static u8 piix_vmw_bmdma_status(struct ata_port *ap);
static int piix_sidpr_scr_read(struct ata_port *ap, unsigned int reg, u32 *val);
static int piix_sidpr_scr_write(struct ata_port *ap, unsigned int reg, u32 val);
+static int piix_qc_defer(struct ata_queued_cmd *qc);
#ifdef CONFIG_PM
static int piix_pci_device_suspend(struct pci_dev *pdev, pm_message_t mesg);
static int piix_pci_device_resume(struct pci_dev *pdev);
@@ -299,6 +300,7 @@ static struct ata_port_operations piix_pata_ops = {
.set_piomode = piix_set_piomode,
.set_dmamode = piix_set_dmamode,
.prereset = piix_pata_prereset,
+ .qc_defer = piix_qc_defer,

static struct ata_port_operations piix_vmw_ops = {
@@ -314,6 +316,7 @@ static struct ata_port_operations ich_pata_ops = {

static struct ata_port_operations piix_sata_ops = {
.inherits = &ata_bmdma_port_ops,
+ .qc_defer = piix_qc_defer,

static struct ata_port_operations piix_sidpr_sata_ops = {
@@ -323,6 +326,40 @@ static struct ata_port_operations piix_sidpr_sata_ops = {
.scr_write = piix_sidpr_scr_write,

+static unsigned int defer_count = 0;
+static struct timer_list defer_timer;
+static void piix_defer_timeout(unsigned long data)
+ struct ata_port *ap = (struct ata_port *)data;
+ spin_lock_bh(ap->lock);
+ defer_count = 0;
+ spin_unlock_bh(ap->lock);
+static int piix_qc_defer(struct ata_queued_cmd *qc)
+ static struct ata_port *ap = NULL;
+ if (!ap) {
+ ap = qc->ap;
+ = (unsigned long)ap;
+ defer_timer.function = piix_defer_timeout;
+ init_timer(&defer_timer);
+ } else if (ap != qc->ap)
+ return 0;
+ defer_count++;
+ if (defer_count < PIIX_QC_DEFER_THRESHOLD)
+ return 0;
+ if (defer_count == PIIX_QC_DEFER_THRESHOLD)
+ mod_timer(&defer_timer, msecs_to_jiffies(5));
+ return ATA_DEFER_LINK;
static const struct piix_map_db ich5_map_db = {
.mask = 0x7,
.port_enable = 0x3,

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