Date:	Wed, 10 Dec 2008 23:56:37 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Theodore Tso <tytso@....edu>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org,
	Christoph Lameter <clameter@....com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

Andrew Morton a écrit :
> On Wed, 10 Dec 2008 06:09:08 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:
> 
>> Now percpu_counter_sum() is 'fixed', what about "percpu_counter_add()" ?
>>
>> void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
>> {
>>         s64 count;
>>         s32 *pcount;
>>         int cpu = get_cpu();
>>
>>         pcount = per_cpu_ptr(fbc->counters, cpu);
>>         count = *pcount + amount;
>>         if (count >= batch || count <= -batch) {
>>                 spin_lock(&fbc->lock);
>>                 fbc->count += count;
>>                 *pcount = 0;
>>                 spin_unlock(&fbc->lock);
>>         } else {
>>                 *pcount = count;
>>         }
>>         put_cpu();
>> }
>>
>>
>> If I read this well, this is not IRQ safe.
> 
> Sure.  It's racy against interrupts on this cpu, it'll deadlock over
> the non-irq-safe spinlock and lockdep will have a coronary over it.
> 
>> get_cpu() only disables preemption IMHO
> 
> yes
> 
>> For nr_files, nr_dentry, nr_inodes, it should not be a problem.
> 
> yes
> 
>> But for network counters (only in net-next-2.6) 
>> and lib/proportions.c, we have a problem ?
> 
> yes
> 
>> Using local_t instead of s32 for cpu 
>> local counter here is possible, so that fast path doesnt have 
>> to disable interrupts
>>
>> (use a local_t instead of s32 for fbc->counters)
>>
>> void __percpu_counter_add_irqsafe(struct percpu_counter *fbc, s64 amount, s32 batch)
>> {
>>         long count;
>>         local_t *pcount;
>>
>> 	/* following code only matters on 32bit arches */
>> 	if (sizeof(amount) != sizeof(local_t)) {
>> 		if (unlikely(amount >= batch || amount <= -batch))) {
>>         	        spin_lock_irqsave(&fbc->lock, flags);
>>                 	fbc->count += amount;
>>                 	spin_unlock_irqrestore(&fbc->lock, flags);
>> 			return;
>> 		}
>> 	}
>>         pcount = per_cpu_ptr(fbc->counters, get_cpu());
>>         count = local_add_return((long)amount, pcount);
>>         if (unlikely(count >= batch || count <= -batch)) {
>> 		unsigned long flags;
>>
>> 		local_sub(count, pcount);
>>                 spin_lock_irqsave(&fbc->lock, flags);
>>                 fbc->count += count;
>>                 spin_unlock_irqrestore(&fbc->lock, flags);
>>         }
>>         put_cpu();
>> }
> 
> 
> I think it's reasonable.  If the batching is working as intended, the
> increased cost of s/spin_lock/spin_lock_irqsave/ should be
> insignificant.
> 
> In fact, if *at all* possible it would be best to make percpu_counters
> irq-safe under all circumstances and avoid fattening and complicating the
> interface.
> 
> 
> 
> But before adding more dependencies on local_t I do think we should
> refresh ourselves on Christoph's objections to them - I remember
> finding them fairly convincing at the time, but I don't recall the
> details.
> 
> <searches for a long time>
> 
> Here, I think:
> http://lkml.indiana.edu/hypermail/linux/kernel/0805.3/2482.html
> 
> Rusty, Christoph: talk to me.  If we add a new user of local_t in core
> kernel, will we regret it?
> 

__percpu_counter_add() already disables preemption (calling get_cpu())


But then, some (all but x86 ;) ) arches dont have true local_t and we fallback
to plain atomic_long_t, and this is wrong because it would add a LOCKED
instruction in fast path.

I remember Christoph added FAST_CMPXCHG_LOCAL, but no more uses of it in current
tree.


Ie : using local_t only if CONFIG_FAST_CMPXCHG_LOCAL, else something like :

void __percpu_counter_add_irqsafe(struct percpu_counter *fbc, s64 amount, s32 batch)
{
        s64 count;
        s32 *pcount = per_cpu_ptr(fbc->counters, get_cpu());
        unsigned long flags;

        local_irq_save(flags);
        count = *pcount + amount;
        if (unlikely(count >= batch || count <= -batch)) {
                spin_lock(&fbc->lock);
                fbc->count += count;
                spin_unlock(&fbc->lock);
                count = 0;
        }
        *pcount = count;
        local_irq_restore(flags);
        put_cpu();
}
EXPORT_SYMBOL(__percpu_counter_add_irqsafe);

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