On Tue, Feb 26, 2013 at 3:26 AM, Srivatsa S. Bhat
<srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Lai,
>
> On 02/25/2013 09:23 PM, Lai Jiangshan wrote:
>> Hi, Srivatsa,
>>
>> The target of the whole patchset is nice for me.
>
> Cool! Thanks :-)
>
>> A question: How did you find out the such usages of
>> "preempt_disable()" and convert them? did all are converted?
>>
>
> Well, I scanned through the source tree for usages which implicitly
> disabled CPU offline and converted them over.
How do you scan? could you show the way you scan the source tree.
I can follow your instructions for double checking.
> Its not limited to uses
> of preempt_disable() alone - even spin_locks, rwlocks, local_irq_disable()
> etc also help disable CPU offline. So I tried to dig out all such uses
> and converted them. However, since the merge window is open, a lot of
> new code is flowing into the tree. So I'll have to rescan the tree to
> see if there are any more places to convert.
I remember some code has such assumption:
preempt_disable() (or something else)
//the code assume that the cpu_online_map can't be changed.
preempt_enable()
It is very hard to find out all such kinds of assumptions and fixes them.
(I notice your code mainly fixes code around send_xxxx())
>
>> And I think the lock is too complex and reinvent the wheel, why don't
>> you reuse the lglock?
>
> lglocks? No way! ;-) See below...
>
>> I wrote an untested draft here.
>>
>> Thanks,
>> Lai
>>
>> PS: Some HA tools(I'm writing one) which takes checkpoints of
>> virtual-machines frequently, I guess this patchset can speedup the
>> tools.
>>
>> From 01db542693a1b7fc6f9ece45d57cb529d9be5b66 Mon Sep 17 00:00:00 2001
>> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> Date: Mon, 25 Feb 2013 23:14:27 +0800
>> Subject: [PATCH] lglock: add read-preference local-global rwlock
>>
>> locality via lglock(trylock)
>> read-preference read-write-lock via fallback rwlock_t
>>
>> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> ---
>> include/linux/lglock.h | 31 +++++++++++++++++++++++++++++++
>> kernel/lglock.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>> 2 files changed, 76 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/lglock.h b/include/linux/lglock.h
>> index 0d24e93..30fe887 100644
>> --- a/include/linux/lglock.h
>> +++ b/include/linux/lglock.h
>> @@ -67,4 +67,35 @@ void lg_local_unlock_cpu(struct lglock *lg, int cpu);
>> void lg_global_lock(struct lglock *lg);
>> void lg_global_unlock(struct lglock *lg);
>>
>> +struct lgrwlock {
>> + unsigned long __percpu *fallback_reader_refcnt;
>> + struct lglock lglock;
>> + rwlock_t fallback_rwlock;
>> +};
>> +
>> +#define DEFINE_LGRWLOCK(name) \
>> + static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock) \
>> + = __ARCH_SPIN_LOCK_UNLOCKED; \
>> + static DEFINE_PER_CPU(unsigned long, name ## _refcnt); \
>> + struct lgrwlock name = { \
>> + .fallback_reader_refcnt = &name ## _refcnt, \
>> + .lglock = { .lock = &name ## _lock } }
>> +
>> +#define DEFINE_STATIC_LGRWLOCK(name) \
>> + static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock) \
>> + = __ARCH_SPIN_LOCK_UNLOCKED; \
>> + static DEFINE_PER_CPU(unsigned long, name ## _refcnt); \
>> + static struct lgrwlock name = { \
>> + .fallback_reader_refcnt = &name ## _refcnt, \
>> + .lglock = { .lock = &name ## _lock } }
>> +
>> +static inline void lg_rwlock_init(struct lgrwlock *lgrw, char *name)
>> +{
>> + lg_lock_init(&lgrw->lglock, name);
>> +}
>> +
>> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw);
>> +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw);
>> +void lg_rwlock_global_write_lock(struct lgrwlock *lgrw);
>> +void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw);
>> #endif
>> diff --git a/kernel/lglock.c b/kernel/lglock.c
>> index 6535a66..463543a 100644
>> --- a/kernel/lglock.c
>> +++ b/kernel/lglock.c
>> @@ -87,3 +87,48 @@ void lg_global_unlock(struct lglock *lg)
>> preempt_enable();
>> }
>> EXPORT_SYMBOL(lg_global_unlock);
>> +
>> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
>> +{
>> + struct lglock *lg = &lgrw->lglock;
>> +
>> + preempt_disable();
>> + if (likely(!__this_cpu_read(*lgrw->fallback_reader_refcnt))) {
>> + if (likely(arch_spin_trylock(this_cpu_ptr(lg->lock)))) {
>> + rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_);
>> + return;
>> + }
>> + read_lock(&lgrw->fallback_rwlock);
>> + }
>> +
>> + __this_cpu_inc(*lgrw->fallback_reader_refcnt);
>> +}
>> +EXPORT_SYMBOL(lg_rwlock_local_read_lock);
>> +
>> +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw)
>> +{
>> + if (likely(!__this_cpu_read(*lgrw->fallback_reader_refcnt))) {
>> + lg_local_unlock(&lgrw->lglock);
>> + return;
>> + }
>> +
>> + if (!__this_cpu_dec_return(*lgrw->fallback_reader_refcnt))
>> + read_unlock(&lgrw->fallback_rwlock);
>> +
>> + preempt_enable();
>> +}
>> +EXPORT_SYMBOL(lg_rwlock_local_read_unlock);
>> +
>
> If I read the code above correctly, all you are doing is implementing a
> recursive reader-side primitive (ie., allowing the reader to call these
> functions recursively, without resulting in a self-deadlock).
>
> But the thing is, making the reader-side recursive is the least of our
> problems! Our main challenge is to make the locking extremely flexible
> and also safe-guard it against circular-locking-dependencies and deadlocks.
> Please take a look at the changelog of patch 1 - it explains the situation
> with an example.
>
>> +void lg_rwlock_global_write_lock(struct lgrwlock *lgrw)
>> +{
>> + lg_global_lock(&lgrw->lglock);
>
> This does a for-loop on all CPUs and takes their locks one-by-one. That's
> exactly what we want to prevent, because that is the _source_ of all our
> deadlock woes in this case. In the presence of perfect lock ordering
> guarantees, this wouldn't have been a problem (that's why lglocks are
> being used successfully elsewhere in the kernel). In the stop-machine()
> removal case, the over-flexibility of preempt_disable() forces us to provide
> an equally flexible locking alternative. Hence we can't use such per-cpu
> locking schemes.
>
> You might note that, for exactly this reason, I haven't actually used any
> per-cpu _locks_ in this synchronization scheme, though it is named as
> "per-cpu rwlocks". The only per-cpu component here are the refcounts, and
> we consciously avoid waiting/spinning on them (because then that would be
> equivalent to having per-cpu locks, which are deadlock-prone). We use
> global rwlocks to get the deadlock-safety that we need.
>
>> + write_lock(&lgrw->fallback_rwlock);
>> +}
>> +EXPORT_SYMBOL(lg_rwlock_global_write_lock);
>> +
>> +void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw)
>> +{
>> + write_unlock(&lgrw->fallback_rwlock);
>> + lg_global_unlock(&lgrw->lglock);
>> +}
>> +EXPORT_SYMBOL(lg_rwlock_global_write_unlock);
>>
>
> Regards,
> Srivatsa S. Bhat
>
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