On Tue, Feb 26, 2013 at 8:17 AM, Lai Jiangshan <eag0628@xxxxxxxxx> wrote: > 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. 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. >> >>> 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. > > > My lock fixes your requirements(I read patch 1-6 before I sent). In s/fixes/fits/ > readsite, lglock 's lock is token via trylock, the lglock doesn't > contribute to deadlocks, we can consider it doesn't exist when we find > deadlock from it. And global fallback rwlock doesn't result to > deadlocks because it is read-preference(you need to inc the > fallback_reader_refcnt inside the cpu-hotplug write-side, I don't do > it in generic lgrwlock) > > > If lg_rwlock_local_read_lock() spins, which means > lg_rwlock_local_read_lock() spins on fallback_rwlock, and which means > lg_rwlock_global_write_lock() took the lgrwlock successfully and > return, and which means lg_rwlock_local_read_lock() will stop spinning > when the write side finished. > > >> >>> +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 >> -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html