From: Lai Jiangshan <eag0628@gmail.com>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Michel Lespinasse <walken@google.com>,
linux-doc@vger.kernel.org, peterz@infradead.org,
fweisbec@gmail.com, linux-kernel@vger.kernel.org,
namhyung@kernel.org, mingo@kernel.org,
linux-arch@vger.kernel.org, linux@arm.linux.org.uk,
xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com,
paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com,
linux-pm@vger.kernel.org, rusty@rustcorp.com.au,
rostedt@goodmis.org, rjw@sisk.pl, vincent.guittot@linaro.org,
tglx@linutronix.de, linux-arm-kernel@lists.infradead.org,
netdev@vger.kernel.org, oleg@redhat.com, sbw@mit.edu,
tj@kernel.org, akpm@linux-foundation.org,
linuxppc-dev@lists.ozlabs.org,
Lai Jiangshan <laijs@cn.fujitsu.com>
Subject: Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks
Date: Tue, 26 Feb 2013 21:34:09 +0800 [thread overview]
Message-ID: <CACvQF53gcuEfgM=L8-mPTt4W3GFMte-w6eyO0iv2CobAN68SNw@mail.gmail.com> (raw)
In-Reply-To: <512BBAD8.8010006@linux.vnet.ibm.com>
On Tue, Feb 26, 2013 at 3:26 AM, Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com> 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@cn.fujitsu.com>
>> 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@cn.fujitsu.com>
>> ---
>> 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
>
next prev parent reply other threads:[~2013-02-26 13:34 UTC|newest]
Thread overview: 115+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-02-18 12:38 [PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug Srivatsa S. Bhat
2013-02-18 12:38 ` [PATCH v6 01/46] percpu_rwlock: Introduce the global reader-writer lock backend Srivatsa S. Bhat
2013-02-18 12:38 ` [PATCH v6 02/46] percpu_rwlock: Introduce per-CPU variables for the reader and the writer Srivatsa S. Bhat
2013-02-18 12:38 ` [PATCH v6 03/46] percpu_rwlock: Provide a way to define and init percpu-rwlocks at compile time Srivatsa S. Bhat
2013-02-18 12:38 ` [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Srivatsa S. Bhat
2013-02-18 15:45 ` Michel Lespinasse
2013-02-18 16:21 ` Srivatsa S. Bhat
2013-02-18 16:31 ` Steven Rostedt
2013-02-18 16:46 ` Srivatsa S. Bhat
2013-02-18 17:56 ` Srivatsa S. Bhat
2013-02-18 18:07 ` Michel Lespinasse
2013-02-18 18:14 ` Srivatsa S. Bhat
2013-02-25 15:53 ` Lai Jiangshan
2013-02-25 19:26 ` Srivatsa S. Bhat
2013-02-26 0:17 ` Lai Jiangshan
2013-02-26 0:19 ` Lai Jiangshan
2013-02-26 9:02 ` Srivatsa S. Bhat
2013-02-26 12:59 ` Lai Jiangshan
2013-02-26 14:22 ` Srivatsa S. Bhat
2013-02-26 16:25 ` Lai Jiangshan
2013-02-26 19:30 ` Srivatsa S. Bhat
2013-02-27 0:33 ` Lai Jiangshan
2013-02-27 21:19 ` Srivatsa S. Bhat
2013-03-01 17:44 ` [PATCH] lglock: add read-preference local-global rwlock Lai Jiangshan
2013-03-01 17:53 ` Tejun Heo
2013-03-01 20:06 ` Srivatsa S. Bhat
2013-03-01 18:28 ` Oleg Nesterov
2013-03-02 12:13 ` Michel Lespinasse
2013-03-02 13:14 ` [PATCH V2] " Lai Jiangshan
2013-03-02 17:11 ` Srivatsa S. Bhat
2013-03-05 15:41 ` Lai Jiangshan
2013-03-05 17:55 ` Srivatsa S. Bhat
2013-03-02 17:20 ` Oleg Nesterov
2013-03-03 17:40 ` Oleg Nesterov
2013-03-05 1:37 ` Michel Lespinasse
2013-03-05 15:27 ` Lai Jiangshan
2013-03-05 16:19 ` Michel Lespinasse
2013-03-05 16:41 ` Oleg Nesterov
2013-03-02 17:06 ` [PATCH] " Oleg Nesterov
2013-03-05 15:54 ` Lai Jiangshan
2013-03-05 16:32 ` Michel Lespinasse
2013-03-05 16:35 ` Oleg Nesterov
2013-03-02 13:42 ` Lai Jiangshan
2013-03-02 17:01 ` Oleg Nesterov
2013-03-01 17:50 ` [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks Lai Jiangshan
2013-03-01 19:47 ` Srivatsa S. Bhat
2013-03-05 16:25 ` Lai Jiangshan
2013-03-05 18:27 ` Srivatsa S. Bhat
2013-03-01 18:10 ` Tejun Heo
2013-03-01 19:59 ` Srivatsa S. Bhat
2013-02-27 11:11 ` Michel Lespinasse
2013-02-27 19:25 ` Oleg Nesterov
2013-02-28 11:34 ` Michel Lespinasse
2013-02-28 18:00 ` Oleg Nesterov
2013-02-28 18:20 ` Oleg Nesterov
2013-02-26 13:34 ` Lai Jiangshan [this message]
2013-02-26 15:17 ` Srivatsa S. Bhat
2013-02-26 14:17 ` Lai Jiangshan
2013-02-26 14:37 ` Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 05/46] percpu_rwlock: Make percpu-rwlocks IRQ-safe, optimally Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 07/46] percpu_rwlock: Allow writers to be readers, and add lockdep annotations Srivatsa S. Bhat
2013-02-18 15:51 ` Michel Lespinasse
2013-02-18 16:31 ` Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 08/46] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Srivatsa S. Bhat
2013-02-18 16:23 ` Michel Lespinasse
2013-02-18 16:43 ` Srivatsa S. Bhat
2013-02-18 17:21 ` Michel Lespinasse
2013-02-18 18:50 ` Srivatsa S. Bhat
2013-02-19 9:40 ` Michel Lespinasse
2013-02-19 9:55 ` Srivatsa S. Bhat
2013-02-19 10:42 ` David Laight
2013-02-19 10:58 ` Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 09/46] CPU hotplug: Convert preprocessor macros to static inline functions Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 10/46] smp, cpu hotplug: Fix smp_call_function_*() to prevent CPU offline properly Srivatsa S. Bhat
2013-02-18 12:39 ` [PATCH v6 11/46] smp, cpu hotplug: Fix on_each_cpu_*() " Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 12/46] sched/timer: Use get/put_online_cpus_atomic() to prevent CPU offline Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 13/46] sched/migration: Use raw_spin_lock/unlock since interrupts are already disabled Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 14/46] sched/rt: Use get/put_online_cpus_atomic() to prevent CPU offline Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 15/46] tick: " Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 16/46] time/clocksource: " Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 17/46] clockevents: Use get/put_online_cpus_atomic() in clockevents_notify() Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 18/46] softirq: Use get/put_online_cpus_atomic() to prevent CPU offline Srivatsa S. Bhat
2013-02-18 12:40 ` [PATCH v6 19/46] irq: " Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 20/46] net: " Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 21/46] block: " Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 22/46] crypto: pcrypt - Protect access to cpu_online_mask with get/put_online_cpus() Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 23/46] infiniband: ehca: Use get/put_online_cpus_atomic() to prevent CPU offline Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 24/46] [SCSI] fcoe: " Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 25/46] staging: octeon: " Srivatsa S. Bhat
2013-02-18 12:41 ` [PATCH v6 26/46] x86: " Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 27/46] perf/x86: " Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 28/46] KVM: Use get/put_online_cpus_atomic() to prevent CPU offline from atomic context Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 29/46] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 30/46] x86/xen: " Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 31/46] alpha/smp: " Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 32/46] blackfin/smp: " Srivatsa S. Bhat
2013-02-18 12:42 ` [PATCH v6 33/46] cris/smp: " Srivatsa S. Bhat
2013-02-18 13:07 ` Jesper Nilsson
2013-02-18 12:43 ` [PATCH v6 34/46] hexagon/smp: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 35/46] ia64: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 36/46] m32r: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 37/46] MIPS: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 38/46] mn10300: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 39/46] parisc: " Srivatsa S. Bhat
2013-02-18 12:43 ` [PATCH v6 40/46] powerpc: " Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 41/46] sh: " Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 42/46] sparc: " Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 43/46] tile: " Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 44/46] cpu: No more __stop_machine() in _cpu_down() Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 45/46] CPU hotplug, stop_machine: Decouple CPU hotplug from stop_machine() in Kconfig Srivatsa S. Bhat
2013-02-18 12:44 ` [PATCH v6 46/46] Documentation/cpu-hotplug: Remove references to stop_machine() Srivatsa S. Bhat
2013-02-22 0:31 ` [PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug Rusty Russell
2013-02-25 21:45 ` Srivatsa S. Bhat
2013-03-01 12:05 ` Vincent Guittot
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