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Date: Sat, 19 Feb 2011 12:07:30 +0800
From: Cypher Wu <cypher.w@...il.com>
To: Chris Metcalf <cmetcalf@...era.com>
Cc: David Miller <davem@...emloft.net>, xiyou.wangcong@...il.com,
linux-kernel@...r.kernel.org, eric.dumazet@...il.com,
netdev@...r.kernel.org
Subject: Re: IGMP and rwlock: Dead ocurred again on TILEPro
On Sat, Feb 19, 2011 at 5:51 AM, Chris Metcalf <cmetcalf@...era.com> wrote:
> On 2/17/2011 10:16 PM, Cypher Wu wrote:
>> On Fri, Feb 18, 2011 at 7:18 AM, Chris Metcalf <cmetcalf@...era.com> wrote:
>>> The interrupt architecture on Tile allows a write to a special-purpose
>>> register to put you into a "critical section" where no interrupts or faults
>>> are delivered. So we just need to bracket the read_lock operations with
>>> two SPR writes; each takes six machine cycles, so we're only adding 12
>>> cycles to the total cost of taking or releasing a read lock on an rwlock
>> I agree that just lock interrupt for read operations should be enough,
>> but read_unlock() is also the place we should lock interrupt, right?
>> If interrupt occurred when it hold lock-val after TNS deadlock still
>> can occur.
>
> Correct; that's what I meant by "read_lock operations". This include lock,
> trylock, and unlock.
>
>> When will you release out that patch? Since time is tight, so maybe
>> I've to fix-up it myself.
>
> I heard from one of our support folks that you were asking through that
> channel, so I asked him to go ahead and give you the spinlock sources
> directly. I will be spending time next week syncing up our internal tree
> with the public git repository so you'll see it on LKML at that time.
>
>> 1. If we use SPR_INTERRUPT_CRITICAL_SECTION it will disable all the
>> interrupt which claimed 'CM', is that right? Should we have to same
>> its original value and restore it later?
>
> We don't need to save and restore, since INTERRUPT_CRITICAL_SECTION is
> almost always zero except in very specific situations.
>
>> 2. Should we lock interrupt for the whole operation of
>> read_lock()/read_unlock(), or we should leave interrupt critical
>> section if it run into __raw_read_lock_slow() and before have to
>> delay_backoff() some time, and re-enter interrupt critical section
>> again before TNS?
>
> Correct, the fix only holds the critical section around the tns and the
> write-back, not during the delay_backoff().
>
>> Bye the way, other RISC platforms, say ARM and MIPS, use store
>> conditional rather that TNS a temp value for lock-val, does Fx have
>> similar instructions?
>
> TILEPro does not have anything more than test-and-set; TILE-Gx (the 64-bit
> processor) has a full array of atomic instructions.
>
>> Adding that to SPR writes should be fine, but it may cause interrupt
>> delay a little more that other platform's read_lock()?
>
> A little, but I think it's in the noise relative to the basic cost of
> read_lock in the absence of full-fledged atomic instructions.
>
>> Another question: What NMI in the former mail means?
>
> Non-maskable interrupt, such as performance counter interrupts.
>
> --
> Chris Metcalf, Tilera Corp.
> http://www.tilera.com
>
>
I've got your source code, thank you very much.
There is still two more question:
1. Why we merge the inlined code and the *_slow into none inlined functions?
2. I've seen the use of 'mb()' in unlock operation, but we don't use
that in the lock operation.
I've released a temporary version with that modification under our
customer' demand, since they want to do a long time test though this
weekend. I'll appreciate that if you gave some comment on my
modifications:
*** /opt/TileraMDE-2.1.0.98943/tilepro/src/sys/linux/include/asm-tile/spinlock_32.h
2010-04-02 11:07:47.000000000 +0800
--- include/asm-tile/spinlock_32.h 2011-02-18 17:09:40.000000000 +0800
***************
*** 12,17 ****
--- 12,27 ----
* more details.
*
* 32-bit SMP spinlocks.
+ *
+ *
+ * The use of TNS instruction cause race condition for system call and
+ * interrupt, so we have to lock interrupt when we trying lock-value.
+ * However, since write_lock() is exclusive so if we really need to
+ * operate it in interrupt then system call have to use write_lock_irqsave(),
+ * So it don't need to lock interrupt here.
+ * Spinlock is also exclusive so we don't take care about it.
+ *
+ * Modified by Cyberman Wu on Feb 18th, 2011.
*/
#ifndef _ASM_TILE_SPINLOCK_32_H
*************** void __raw_read_unlock_slow(raw_rwlock_t
*** 86,91 ****
--- 96,114 ----
void __raw_write_lock_slow(raw_rwlock_t *, u32);
void __raw_write_unlock_slow(raw_rwlock_t *, u32);
+
+ static inline void __raw_read_lock_enter_critical(void)
+ {
+ BUG_ON(__insn_mfspr(SPR_INTERRUPT_CRITICAL_SECTION));
+ __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
+ }
+
+ static inline void __raw_read_lock_leave_critical(void)
+ {
+ __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
+ }
+
+
/**
* __raw_read_can_lock() - would read_trylock() succeed?
*/
*************** static inline int __raw_write_can_lock(r
*** 107,121 ****
*/
static inline void __raw_read_lock(raw_rwlock_t *rwlock)
{
! u32 val = __insn_tns((int *)&rwlock->lock);
if (unlikely(val << _RD_COUNT_WIDTH)) {
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
__raw_read_lock_slow(rwlock, val);
return;
}
rwlock->lock = val + (1 << _RD_COUNT_SHIFT);
}
/**
--- 130,148 ----
*/
static inline void __raw_read_lock(raw_rwlock_t *rwlock)
{
! u32 val;
! __raw_read_lock_enter_critical();
! /*u32 */val = __insn_tns((int *)&rwlock->lock);
if (unlikely(val << _RD_COUNT_WIDTH)) {
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
__raw_read_lock_slow(rwlock, val);
+ __raw_read_lock_leave_critical();
return;
}
rwlock->lock = val + (1 << _RD_COUNT_SHIFT);
+ __raw_read_lock_leave_critical();
}
/**
*************** static inline void __raw_write_lock(raw_
*** 140,154 ****
static inline int __raw_read_trylock(raw_rwlock_t *rwlock)
{
int locked;
! u32 val = __insn_tns((int *)&rwlock->lock);
if (unlikely(val & 1)) {
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
! return __raw_read_trylock_slow(rwlock);
}
locked = (val << _RD_COUNT_WIDTH) == 0;
rwlock->lock = val + (locked << _RD_COUNT_SHIFT);
return locked;
}
--- 167,187 ----
static inline int __raw_read_trylock(raw_rwlock_t *rwlock)
{
int locked;
! u32 val;
! __raw_read_lock_enter_critical();
! /*u32 */val = __insn_tns((int *)&rwlock->lock);
if (unlikely(val & 1)) {
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
! // return __raw_read_trylock_slow(rwlock);
! locked =__raw_read_trylock_slow(rwlock);
! __raw_read_lock_leave_critical();
! return locked;
}
locked = (val << _RD_COUNT_WIDTH) == 0;
rwlock->lock = val + (locked << _RD_COUNT_SHIFT);
+ __raw_read_lock_leave_critical();
return locked;
}
*************** static inline void __raw_read_unlock(raw
*** 184,198 ****
--- 217,234 ----
{
u32 val;
mb();
+ __raw_read_lock_enter_critical();
val = __insn_tns((int *)&rwlock->lock);
if (unlikely(val & 1)) {
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
__raw_read_unlock_slow(rwlock);
+ __raw_read_lock_leave_critical();
return;
}
rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
+ __raw_read_lock_leave_critical();
}
--- /opt/TileraMDE-2.1.0.98943/tilepro/src/sys/linux/arch/tile/lib/spinlock_32.c
2010-04-02 11:08:02.000000000 +0800
+++ arch/tile/lib/spinlock_32.c 2011-02-18 16:05:31.000000000 +0800
@@ -98,7 +98,18 @@ static inline u32 get_rwlock(raw_rwlock_
#ifdef __TILECC__
#pragma frequency_hint NEVER
#endif
+ /*
+ * get_rwlock() now have to be called in Interrupt
+ * Critical Section, so it can't be called in the
+ * these __raw_write_xxx() anymore!!!!!
+ *
+ * We leave Interrupt Critical Section for making
+ * interrupt delay minimal.
+ * Is that really needed???
+ */
+ __raw_read_lock_leave_critical();
delay_backoff(iterations++);
+ __raw_read_lock_enter_critical();
continue;
}
return val;
@@ -152,7 +163,14 @@ void __raw_read_lock_slow(raw_rwlock_t *
do {
if (!(val & 1))
rwlock->lock = val;
+ /*
+ * We leave Interrupt Critical Section for making
+ * interrupt delay minimal.
+ * Is that really needed???
+ */
+ __raw_read_lock_leave_critical();
delay_backoff(iterations++);
+ __raw_read_lock_enter_critical();
val = __insn_tns((int *)&rwlock->lock);
} while ((val << RD_COUNT_WIDTH) != 0);
rwlock->lock = val + (1 << RD_COUNT_SHIFT);
@@ -166,23 +184,30 @@ void __raw_write_lock_slow(raw_rwlock_t
* when we compare them.
*/
u32 my_ticket_;
+ u32 iterations = 0;
- /* Take out the next ticket; this will also stop would-be readers. */
- if (val & 1)
- val = get_rwlock(rwlock);
- rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
+ /*
+ * Wait until there are no readers, then bump up the next
+ * field and capture the ticket value.
+ */
+ for (;;) {
+ if (!(val & 1)) {
+ if ((val >> RD_COUNT_SHIFT) == 0)
+ break;
+ rwlock->lock = val;
+ }
+ delay_backoff(iterations++);
+ val = __insn_tns((int *)&rwlock->lock);
+ }
- /* Extract my ticket value from the original word. */
+ /* Take out the next ticket and extract my ticket value. */
+ rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
my_ticket_ = val >> WR_NEXT_SHIFT;
- /*
- * Wait until the "current" field matches our ticket, and
- * there are no remaining readers.
- */
+ /* Wait until the "current" field matches our ticket. */
for (;;) {
u32 curr_ = val >> WR_CURR_SHIFT;
- u32 readers = val >> RD_COUNT_SHIFT;
- u32 delta = ((my_ticket_ - curr_) & WR_MASK) + !!readers;
+ u32 delta = ((my_ticket_ - curr_) & WR_MASK);
if (likely(delta == 0))
break;
--
Cyberman Wu
--
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