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SubjectHardlock with all kernel >= 2.4.7-pre5 - softirq related ?
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Date
I have experienced hardlocks (everything freezes with small white square
box with pointer in upper left corner on screen and only SysRq-b seems
to work) without any oops or anything.

I have after some trial and error narrowed it down to the following part
of the interdiff between 2.4.7-pre4 and 2.4.7-pre5:
(For config, lspci -vvv, program versions and boot logs see
http://thsv.dnsalias.org/~thsv/lock/config
http://thsv.dnsalias.org/~thsv/lock/lspci-vvv
http://thsv.dnsalias.org/~thsv/lock/versions
http://thsv.dnsalias.org/~thsv/lock/messages
http://thsv.dnsalias.org/~thsv/lock/boot.log )

--- linux/kernel/ksyms.c
+++ linux/kernel/ksyms.c
@@ -540,0 +541,2 @@
+EXPORT_SYMBOL(raise_softirq);
+EXPORT_SYMBOL(cpu_raise_softirq);
--- v2.4.6/linux/kernel/softirq.c Tue Jul 3 17:08:22 2001
+++ linux/kernel/softirq.c Mon Jul 9 16:32:45 2001
@@ -47,21 +47,38 @@

static struct softirq_action softirq_vec[32] __cacheline_aligned;

+/*
+ * we cannot loop indefinitely here to avoid userspace starvation,
+ * but we also don't want to introduce a worst case 1/HZ latency
+ * to the pending events, so lets the scheduler to balance
+ * the softirq load for us.
+ */
+static inline void wakeup_softirqd(unsigned cpu)
+{
+ struct task_struct * tsk = ksoftirqd_task(cpu);
+
+ if (tsk && tsk->state != TASK_RUNNING)
+ wake_up_process(tsk);
+}
+
asmlinkage void do_softirq()
{
int cpu = smp_processor_id();
__u32 pending;
+ long flags;
+ __u32 mask;

if (in_interrupt())
return;

- local_irq_disable();
+ local_irq_save(flags);

pending = softirq_pending(cpu);

if (pending) {
struct softirq_action *h;

+ mask = ~pending;
local_bh_disable();
restart:
/* Reset the pending bitmask before enabling irqs */
@@ -81,14 +98,40 @@
local_irq_disable();

pending = softirq_pending(cpu);
- if (pending)
+ if (pending & mask) {
+ mask &= ~pending;
goto restart;
+ }
__local_bh_enable();
+
+ if (pending)
+ wakeup_softirqd(cpu);
}

- local_irq_enable();
+ local_irq_restore(flags);
}

+inline void cpu_raise_softirq(unsigned int cpu, unsigned int nr)
+{
+ __cpu_raise_softirq(cpu, nr);
+
+ /*
+ * If we're in an interrupt or bh, we're done
+ * (this also catches bh-disabled code). We will
+ * actually run the softirq once we return from
+ * the irq or bh.
+ *
+ * Otherwise we wake up ksoftirqd to make sure we
+ * schedule the softirq soon.
+ */
+ if (!(local_irq_count(cpu) | local_bh_count(cpu)))
+ wakeup_softirqd(cpu);
+}
+
+void raise_softirq(unsigned int nr)
+{
+ cpu_raise_softirq(smp_processor_id(), nr);
+}

void open_softirq(int nr, void (*action)(struct softirq_action*), void *data)
{
@@ -112,11 +155,10 @@
* If nobody is running it then add it to this CPU's
* tasklet queue.
*/
- if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state) &&
- tasklet_trylock(t)) {
+ if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
t->next = tasklet_vec[cpu].list;
tasklet_vec[cpu].list = t;
- __cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
+ cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
tasklet_unlock(t);
}
local_irq_restore(flags);
@@ -130,11 +172,10 @@
cpu = smp_processor_id();
local_irq_save(flags);

- if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state) &&
- tasklet_trylock(t)) {
+ if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
t->next = tasklet_hi_vec[cpu].list;
tasklet_hi_vec[cpu].list = t;
- __cpu_raise_softirq(cpu, HI_SOFTIRQ);
+ cpu_raise_softirq(cpu, HI_SOFTIRQ);
tasklet_unlock(t);
}
local_irq_restore(flags);
@@ -148,37 +189,30 @@
local_irq_disable();
list = tasklet_vec[cpu].list;
tasklet_vec[cpu].list = NULL;
+ local_irq_enable();

while (list) {
struct tasklet_struct *t = list;

list = list->next;

- /*
- * A tasklet is only added to the queue while it's
- * locked, so no other CPU can have this tasklet
- * pending:
- */
if (!tasklet_trylock(t))
BUG();
-repeat:
- if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
- BUG();
if (!atomic_read(&t->count)) {
- local_irq_enable();
+ if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+ BUG();
t->func(t->data);
- local_irq_disable();
- /*
- * One more run if the tasklet got reactivated:
- */
- if (test_bit(TASKLET_STATE_SCHED, &t->state))
- goto repeat;
+ tasklet_unlock(t);
+ continue;
}
tasklet_unlock(t);
- if (test_bit(TASKLET_STATE_SCHED, &t->state))
- tasklet_schedule(t);
+
+ local_irq_disable();
+ t->next = tasklet_vec[cpu].list;
+ tasklet_vec[cpu].list = t;
+ cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
+ local_irq_enable();
}
- local_irq_enable();
}


@@ -193,6 +227,7 @@
local_irq_disable();
list = tasklet_hi_vec[cpu].list;
tasklet_hi_vec[cpu].list = NULL;
+ local_irq_enable();

while (list) {
struct tasklet_struct *t = list;
@@ -201,21 +236,21 @@

if (!tasklet_trylock(t))
BUG();
-repeat:
- if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
- BUG();
if (!atomic_read(&t->count)) {
- local_irq_enable();
+ if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+ BUG();
t->func(t->data);
- local_irq_disable();
- if (test_bit(TASKLET_STATE_SCHED, &t->state))
- goto repeat;
+ tasklet_unlock(t);
+ continue;
}
tasklet_unlock(t);
- if (test_bit(TASKLET_STATE_SCHED, &t->state))
- tasklet_hi_schedule(t);
+
+ local_irq_disable();
+ t->next = tasklet_hi_vec[cpu].list;
+ tasklet_hi_vec[cpu].list = t;
+ cpu_raise_softirq(cpu, HI_SOFTIRQ);
+ local_irq_enable();
}
- local_irq_enable();
}


@@ -335,3 +370,61 @@
f(data);
}
}
+
+static int ksoftirqd(void * __bind_cpu)
+{
+ int bind_cpu = *(int *) __bind_cpu;
+ int cpu = cpu_logical_map(bind_cpu);
+
+ daemonize();
+ current->nice = 19;
+ sigfillset(&current->blocked);
+
+ /* Migrate to the right CPU */
+ current->cpus_allowed = 1UL << cpu;
+ while (smp_processor_id() != cpu)
+ schedule();
+
+ sprintf(current->comm, "ksoftirqd_CPU%d", bind_cpu);
+
+ __set_current_state(TASK_INTERRUPTIBLE);
+ mb();
+
+ ksoftirqd_task(cpu) = current;
+
+ for (;;) {
+ if (!softirq_pending(cpu))
+ schedule();
+
+ __set_current_state(TASK_RUNNING);
+
+ while (softirq_pending(cpu)) {
+ do_softirq();
+ if (current->need_resched)
+ schedule();
+ }
+
+ __set_current_state(TASK_INTERRUPTIBLE);
+ }
+}
+
+static __init int spawn_ksoftirqd(void)
+{
+ int cpu;
+
+ for (cpu = 0; cpu < smp_num_cpus; cpu++) {
+ if (kernel_thread(ksoftirqd, (void *) &cpu,
+ CLONE_FS | CLONE_FILES | CLONE_SIGNAL) < 0)
+ printk("spawn_ksoftirqd() failed for cpu %d\n", cpu);
+ else {
+ while (!ksoftirqd_task(cpu_logical_map(cpu))) {
+ current->policy |= SCHED_YIELD;
+ schedule();
+ }
+ }
+ }
+
+ return 0;
+}
+
+__initcall(spawn_ksoftirqd);
--- v2.4.6/linux/kernel/sched.c Tue Jul 3 17:08:22 2001
+++ linux/kernel/sched.c Mon Jul 9 17:51:59 2001
@@ -543,11 +543,6 @@

release_kernel_lock(prev, this_cpu);

- /* Do "administrative" work here while we don't hold any locks */
- if (softirq_pending(this_cpu))
- goto handle_softirq;
-handle_softirq_back:
-
/*
* 'sched_data' is protected by the fact that we can run
* only one process per CPU.
@@ -689,13 +684,11 @@
goto repeat_schedule;

still_running:
+ if (!(prev->cpus_allowed & (1UL << this_cpu)))
+ goto still_running_back;
c = goodness(prev, this_cpu, prev->active_mm);
next = prev;
goto still_running_back;
-
-handle_softirq:
- do_softirq();
- goto handle_softirq_back;

move_rr_last:
if (!prev->counter) {
--- v2.4.6/linux/include/linux/netdevice.h Fri May 25 18:02:01 2001
+++ linux/include/linux/netdevice.h Mon Jul 9 18:34:08 2001
@@ -487,7 +487,7 @@
local_irq_save(flags);
dev->next_sched = softnet_data[cpu].output_queue;
softnet_data[cpu].output_queue = dev;
- __cpu_raise_softirq(cpu, NET_TX_SOFTIRQ);
+ cpu_raise_softirq(cpu, NET_TX_SOFTIRQ);
local_irq_restore(flags);
}
}
@@ -536,7 +536,7 @@
local_irq_save(flags);
skb->next = softnet_data[cpu].completion_queue;
softnet_data[cpu].completion_queue = skb;
- __cpu_raise_softirq(cpu, NET_TX_SOFTIRQ);
+ cpu_raise_softirq(cpu, NET_TX_SOFTIRQ);
local_irq_restore(flags);
}
}
--- v2.4.6/linux/include/linux/irq_cpustat.h Tue Jul 3 17:08:21 2001
+++ linux/include/linux/irq_cpustat.h Mon Jul 9 18:33:25 2001
@@ -30,6 +30,7 @@
#define local_irq_count(cpu) __IRQ_STAT((cpu), __local_irq_count)
#define local_bh_count(cpu) __IRQ_STAT((cpu), __local_bh_count)
#define syscall_count(cpu) __IRQ_STAT((cpu), __syscall_count)
+#define ksoftirqd_task(cpu) __IRQ_STAT((cpu), __ksoftirqd_task)
/* arch dependent irq_stat fields */
#define nmi_count(cpu) __IRQ_STAT((cpu), __nmi_count) /* i386, ia64 */

--- v2.4.6/linux/include/linux/interrupt.h Tue Jul 3 17:08:21 2001
+++ linux/include/linux/interrupt.h Mon Jul 9 18:33:25 2001
@@ -73,8 +73,9 @@

asmlinkage void do_softirq(void);
extern void open_softirq(int nr, void (*action)(struct softirq_action*), void *data);
-
extern void softirq_init(void);
+extern void FASTCALL(cpu_raise_softirq(unsigned int cpu, unsigned int nr));
+extern void FASTCALL(raise_softirq(unsigned int nr));



@@ -129,7 +130,7 @@
extern struct tasklet_head tasklet_hi_vec[NR_CPUS];

#define tasklet_trylock(t) (!test_and_set_bit(TASKLET_STATE_RUN, &(t)->state))
-#define tasklet_unlock(t) clear_bit(TASKLET_STATE_RUN, &(t)->state)
+#define tasklet_unlock(t) do { smp_mb__before_clear_bit(); clear_bit(TASKLET_STATE_RUN, &(t)->state); } while(0)
#define tasklet_unlock_wait(t) while (test_bit(TASKLET_STATE_RUN, &(t)->state)) { barrier(); }

extern void tasklet_schedule(struct tasklet_struct *t);
--- v2.4.6/linux/include/asm-i386/softirq.h Tue Jul 3 17:08:21 2001
+++ linux/include/asm-i386/softirq.h Mon Jul 9 18:33:25 2001
@@ -11,8 +11,6 @@

#define local_bh_disable() cpu_bh_disable(smp_processor_id())
#define __local_bh_enable() __cpu_bh_enable(smp_processor_id())
-#define __cpu_raise_softirq(cpu,nr) set_bit((nr), &softirq_pending(cpu));
-#define raise_softirq(nr) __cpu_raise_softirq(smp_processor_id(), (nr))

#define in_softirq() (local_bh_count(smp_processor_id()) != 0)

@@ -28,6 +26,7 @@
do { \
unsigned int *ptr = &local_bh_count(smp_processor_id()); \
\
+ barrier(); \
if (!--*ptr) \
__asm__ __volatile__ ( \
"cmpl $0, -8(%0);" \
@@ -45,5 +44,7 @@
: "r" (ptr), "i" (do_softirq) \
/* no registers clobbered */ ); \
} while (0)
+
+#define __cpu_raise_softirq(cpu, nr) __set_bit(nr, &softirq_pending(cpu))

#endif /* __ASM_SOFTIRQ_H */
--- v2.4.6/linux/include/asm-i386/hardirq.h Tue Jul 3 17:08:21 2001
+++ linux/include/asm-i386/hardirq.h Mon Jul 9 18:33:25 2001
@@ -11,6 +11,7 @@
unsigned int __local_irq_count;
unsigned int __local_bh_count;
unsigned int __syscall_count;
+ struct task_struct * __ksoftirqd_task; /* waitqueue is too large */
unsigned int __nmi_count; /* arch dependent */
} ____cacheline_aligned irq_cpustat_t;


/ Thomas
.......................................................................
Thomas Svedberg
Department of Mathematics
Chalmers University of Technology
Address: S-412 96 Göteborg, SWEDEN
E-mail : thsv@bigfoot.com, thsv@math.chalmers.se
Phone : +46 31 772 5368
Fax : +46 31 772 3595
.......................................................................
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