linux-kernel - Re: [RT PATCH] locking/rwsem-rt: Remove might_sleep() in __up

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Message-ID: <20210708195125.zxqsxy5bpccgxb2n@halaneylaptop>
Date:   Thu, 8 Jul 2021 14:51:25 -0500
From:   Andrew Halaney <ahalaney@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     bigeasy@...utronix.de, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, Chunyu Hu <chuhu@...hat.com>
Subject: Re: [RT PATCH] locking/rwsem-rt: Remove might_sleep() in __up_read()

On Mon, Apr 26, 2021 at 08:16:34PM +0200, Thomas Gleixner wrote:
> On Mon, Apr 26 2021 at 11:42, Andrew Halaney wrote:
> > On Tue, Apr 06, 2021 at 05:19:52PM -0500, Andrew Halaney wrote:
> >> There's no chance of sleeping here, the reader is giving up the
> >> lock and possibly waking up the writer who is waiting on it.
> >> 
> >> Reported-by: Chunyu Hu <chuhu@...hat.com>
> >> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
> >> ---
> >> Hello,
> >> 
> >> I ran into a warning caused by this, and I think the warning is
> >> incorrect. Please let me know if I'm wrong!
> >> I'm working off of linux-5.12.y-rt, but this applies cleanly to older
> >> stable branches as well.
> >> 
> >> Thanks,
> >> Andrew
> >> 
> >>  kernel/locking/rwsem-rt.c | 1 -
> >>  1 file changed, 1 deletion(-)
> >> 
> >> diff --git a/kernel/locking/rwsem-rt.c b/kernel/locking/rwsem-rt.c
> >> index 274172d5bb3a..b61edc4dcb73 100644
> >> --- a/kernel/locking/rwsem-rt.c
> >> +++ b/kernel/locking/rwsem-rt.c
> >> @@ -198,7 +198,6 @@ void __up_read(struct rw_semaphore *sem)
> >>  	if (!atomic_dec_and_test(&sem->readers))
> >>  		return;
> >>  
> >> -	might_sleep();
> >>  	raw_spin_lock_irq(&m->wait_lock);
> >>  	/*
> >>  	 * Wake the writer, i.e. the rtmutex owner. It might release the
> >> -- 
> >> 2.30.2
> >
> > Just a gentle follow up, any feedback?
> 
> Looks correct. Will go into the next rt-release.
> 
> Thanks,
> 
>         tglx
> 

Hi Thomas,

I see a new release (v5.13-rt1) was created, and with it rwsem
was overhauled entirely making this patch pointless for linux-rt-devel.

That being said, it's a little unclear to me how RT only patches
make their way to the maintained branches over in linux-stable-rt.
I think it should be applied to:
 v4.9-rt
 v4.14-rt
 v4.19-rt
 v5.4-rt
 v5.10-rt
to remove the incorrect warning, unless those branches plan to backport
the latest RT patchset with the new rwsem implementation. Is there a
proper way for me signal that?

Thanks,
Andrew