linux-kernel - RE: [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to prevent lock starvation

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Date:   Fri, 3 May 2019 14:37:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>
CC:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Davidlohr Bueso" <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: RE: [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to
 prevent lock starvation

From: Peter Zijlstra
> Sent: 03 May 2019 14:11
> To: Waiman Long
...
> I've changed that like so.
> 
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -406,19 +406,23 @@ static inline bool rwsem_try_write_lock(
>  	long new;
> 
>  	lockdep_assert_held(&sem->wait_lock);
> +
>  	do {
>  		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
> 
>  		if (has_handoff && wstate == WRITER_NOT_FIRST)
>  			return false;
> 
> +		new = count;
> +
>  		if (count & RWSEM_LOCK_MASK) {
>  			if (has_handoff || (wstate != WRITER_HANDOFF))
>  				return false;
> -			new = count | RWSEM_FLAG_HANDOFF;
> +
> +			new |= RWSEM_FLAG_HANDOFF;
>  		} else {
> -			new = (count | RWSEM_WRITER_LOCKED) &
> -				~RWSEM_FLAG_HANDOFF;
> +			new |= RWSEM_WRITER_LOCKED;
> +			new &= ~RWSEM_FLAG_HANDOFF;
> 
>  			if (list_is_singular(&sem->wait_list))
>  				new &= ~RWSEM_FLAG_WAITERS;

The compiler will probably convert it back :-)

	David

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