linux-kernel - RE: [PATCH 2/3] rwsem: Implement down_read

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Date:   Mon, 7 Dec 2020 16:58:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Waiman Long' <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Oleg Nesterov <oleg@...hat.com>,
        "Christopher Yeoh" <cyeoh@....ibm.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        "Sargun Dhillon" <sargun@...gun.me>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Arnd Bergmann <arnd@...db.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: RE: [PATCH 2/3] rwsem: Implement down_read_interruptible

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@...hat.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

	David

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