From: Maxim Kuvyrkov <maxim@codesourcery.com>
To: Andrew Haley <aph@redhat.com>
Cc: David Miller <davem@davemloft.net>, <roland@hack.frob.com>,
<joseph@codesourcery.com>, <rdsandiford@googlemail.com>,
<libc-ports@sourceware.org>, <libc-alpha@sourceware.org>
Subject: Re: [PATCH] Unify pthread_spin_[try]lock implementations.
Date: Wed, 11 Jul 2012 08:54:00 -0000 [thread overview]
Message-ID: <84304C03-6A49-4263-9016-05486EDC0E98@codesourcery.com> (raw)
In-Reply-To: <4FFD3CD9.4030206@redhat.com>
On 11/07/2012, at 8:44 PM, Andrew Haley wrote:
> On 07/11/2012 09:25 AM, David Miller wrote:
>> From: Roland McGrath <roland@hack.frob.com>
>> Date: Wed, 11 Jul 2012 01:14:41 -0700 (PDT)
>>
>>>> +int
>>>> +pthread_spin_lock (pthread_spinlock_t *lock)
>>>> +{
>>>> + while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0)
>>>> + while (*lock != 0)
>>>> + ;
>>>
>>> What's the inner loop for?
>>
>> I guess the idea is to spin with non-atomic reads when the lock is
>> contended so we don't do expensive bus cycles grabbing the cache line
>> in exclusive state over and over again.
>>
>> If we spun using only the atomic it would be very expensive.
>
> Sure, but on ARM at least there's no guarantee that the local processor
> will see changes to the state of the lock when another processor frees
> it.
Hm, but this is exactly what ARM port does and did for a while. I guess the memory on the local processor eventually gets updated and the loop breaks.
This is an interesting point and maybe introducing a counter like below will improve spinlocks for ARM.
+ int counter = 123456;
+ while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0)
+ while (*lock != 0 && --counter)
+ ;
--
Maxim Kuvyrkov
CodeSourcery / Mentor Graphics
next prev parent reply other threads:[~2012-07-11 8:54 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-14 4:36 [PATCH 2/3, MIPS] Rewrite MIPS' pthread_spin_[try]lock using __atomic_* builtins Maxim Kuvyrkov
2012-06-28 23:10 ` Joseph S. Myers
2012-07-11 5:58 ` [PATCH] Unify pthread_spin_[try]lock implementations Maxim Kuvyrkov
2012-07-11 8:15 ` Roland McGrath
2012-07-11 8:25 ` David Miller
2012-07-11 8:44 ` Andrew Haley
2012-07-11 8:54 ` Maxim Kuvyrkov [this message]
2012-07-11 9:02 ` Andrew Haley
2012-07-11 9:05 ` Maxim Kuvyrkov
2012-07-11 11:22 ` Roland McGrath
2012-07-11 14:52 ` Chris Metcalf
2012-07-11 22:16 ` Maxim Kuvyrkov
2012-07-25 18:13 ` Roland McGrath
2012-07-25 19:04 ` Chris Metcalf
2012-07-25 20:22 ` Roland McGrath
2012-07-25 20:29 ` Chris Metcalf
2012-07-25 20:43 ` Roland McGrath
2012-08-15 3:17 ` Maxim Kuvyrkov
2012-08-15 16:27 ` Roland McGrath
2012-08-15 16:39 ` Maxim Kuvyrkov
2012-08-15 16:44 ` Roland McGrath
2012-08-15 16:53 ` Maxim Kuvyrkov
2012-08-15 16:56 ` Roland McGrath
2012-08-15 17:05 ` Maxim Kuvyrkov
2012-08-15 17:05 ` Jeff Law
2012-08-15 17:11 ` Roland McGrath
2012-08-15 17:24 ` Jeff Law
2012-08-15 16:40 ` Joseph S. Myers
2012-08-15 16:43 ` Carlos O'Donell
2012-08-15 22:00 ` Andreas Schwab
2012-08-16 10:21 ` Torvald Riegel
2012-08-16 12:40 ` Chris Metcalf
2012-08-22 23:16 ` Maxim Kuvyrkov
2012-07-11 8:26 ` Maxim Kuvyrkov
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