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Subject: Re: [PATCH 2/3] rwsem: Implement down_read_interruptible
To:     David Laight <David.Laight@ACULAB.COM>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Jann Horn <jannh@google.com>,
        Vasiliy Kulikov <segoon@openwall.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Christopher Yeoh <cyeoh@au1.ibm.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <aef54faf-cead-403c-6088-ff52ce1a5dde@redhat.com>
Date:   Mon, 7 Dec 2020 14:02:25 -0500
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On 12/7/20 11:58 AM, David Laight wrote:
> From: Waiman Long
>> Sent: 07 December 2020 15:34
>>
>> On 12/7/20 4:02 AM, Peter Zijlstra wrote:
>>> On Thu, Dec 03, 2020 at 08:59:13PM -0500, Waiman Long wrote:
>>>> On 12/3/20 3:11 PM, Eric W. Biederman wrote:
>>>>> +static inline int __down_read_interruptible(struct rw_semaphore *sem)
>>>>> +{
>>>>> +	if (!rwsem_read_trylock(sem)) {
>>>>> +		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_INTERRUPTIBLE)))
>>>>> +			return -EINTR;
>>>>> +		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
>>>>> +	} else {
>>>>> +		rwsem_set_reader_owned(sem);
>>>>> +	}
>>>>> +	return 0;
>>>>> +}
>>>>> +
>>>>>     static inline int __down_read_killable(struct rw_semaphore *sem)
>>>>>     {
>>>>>     	if (!rwsem_read_trylock(sem)) {
>>>>> @@ -1495,6 +1507,20 @@ void __sched down_read(struct rw_semaphore *sem)
>>>>>     }
>>>>>     EXPORT_SYMBOL(down_read);
>>>>> +int __sched down_read_interruptible(struct rw_semaphore *sem)
>>>>> +{
>>>>> +	might_sleep();
>>>>> +	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
>>>>> +
>>>>> +	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
>>>>> +		rwsem_release(&sem->dep_map, _RET_IP_);
>>>>> +		return -EINTR;
>>>>> +	}
>>>>> +
>>>>> +	return 0;
>>>>> +}
>>>>> +EXPORT_SYMBOL(down_read_interruptible);
>>>>> +
>>>>>     int __sched down_read_killable(struct rw_semaphore *sem)
>>>>>     {
>>>>>     	might_sleep();
>>>> Acked-by: Waiman Long <longman@redhat.com>
>>> Yeah, that seems correct.. There's an unfortunate amount of copy-paste
>>> there though.
>>>
>>> Do we want to follow that up with something like this?
>> I am actually thinking about similar streamlining once the patch lands.
>>
>> Your suggested changes look fine to me.
> How much more difficult would it be to also add a timeout option?
> I looked at adding one to the mutex code - and fell into a big pile
> of replicated code.
>
> ISTM that one the initial locked exchange (and spin) fails a few
> extra instructions when heading for the sleep don't really matter
>
Actually, I had tried that before. See

https://lore.kernel.org/lkml/20190911150537.19527-1-longman@redhat.com/

That is for rwsem, but the same can be done for mutex. However, Peter 
didn't seem to like the idea of a timeout parameter. Anyway, it is 
certainly doable if there is a good use case for it.

Cheers,
Longman