linux-kernel - Re: [PATCH 3/4] locking/osq_lock: From x_osq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <afddc02d-fa96-4793-bc60-6c81629d8d5b@redhat.com>
Date: Sun, 15 Sep 2024 17:06:10 -0400
From: Waiman Long <longman@...hat.com>
To: Uros Bizjak <ubizjak@...il.com>, yongli-oc <yongli-oc@...oxin.com>,
 peterz@...radead.org, mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc: linux-kernel@...r.kernel.org, yongli@...oxin.com, louisqi@...oxin.com,
 cobechen@...oxin.com, jiangbowang@...oxin.com
Subject: Re: [PATCH 3/4] locking/osq_lock: From x_osq_lock/unlock to
 numa-aware lock/unlock.

On 9/15/24 14:44, Uros Bizjak wrote:
>
>
> On 14. 09. 24 10:53, yongli-oc wrote:
>> According to the contention level, switches from x_osq_lock to
>> numa-aware osq_lock.
>> The numa-aware lock is a two level osq_lock.
>> The Makefile for dynamic numa-aware osq lock.
>>
>> Signed-off-by: yongli-oc <yongli-oc@...oxin.com>
>> ---
>>   kernel/locking/Makefile      |   1 +
>>   kernel/locking/numa.h        |  98 ++++++++
>>   kernel/locking/numa_osq.h    |  32 +++
>>   kernel/locking/x_osq_lock.c  | 332 +++++++++++++++++++++++++++
>>   kernel/locking/zx_numa_osq.c | 433 +++++++++++++++++++++++++++++++++++
>>   5 files changed, 896 insertions(+)
>>   create mode 100644 kernel/locking/numa.h
>>   create mode 100644 kernel/locking/numa_osq.h
>>   create mode 100644 kernel/locking/x_osq_lock.c
>>   create mode 100644 kernel/locking/zx_numa_osq.c
>
> ...
>
>> +    if (lock->numa_enable == OSQLOCKSTOPPING && old == 
>> OSQ_UNLOCKED_VAL)
>> +        old = OSQ_LOCKED_VAL;
>> +
>> +    for (;;) {
>> +        if (READ_ONCE(lock->tail16) == curr &&
>> +            cmpxchg(&lock->tail16, curr, old) == curr) {
>
> I would like to ask if there is any benefit to read the location two 
> times? cmpxchg() reads the location and skips the update when curr is 
> different from the value at the location by itself. Using 
> try_cmpxchg() can produce even more optimized code, since on x86 arch 
> CMPXCHG also sets ZF flag when operand 2 is equal to the value at 
> location (and update happens), and this flag can be used in a 
> conditional jump.

The major reason is for doing a read first before cmpxchg() is to avoid 
the overhead of an atomic operation in case the current task isn't the 
tail. We usually optimize for the case with a lot of incoming lockers 
where the chance of a match isn't particularly high.

Cheers,
Longman