linux-kernel - Re: sem_lock() vs qspinlocks

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Message-ID: <20160522093828.GM3193@twins.programming.kicks-ass.net>
Date:	Sun, 22 May 2016 11:38:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>, Waiman.Long@....com,
	mingo@...nel.org, torvalds@...ux-foundation.org,
	ggherdovich@...e.com, mgorman@...hsingularity.net,
	linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>
Subject: Re: sem_lock() vs qspinlocks

On Sun, May 22, 2016 at 10:43:08AM +0200, Manfred Spraul wrote:
> How would we handle mixed spin_lock()/mutex_lock() code?
> For the IPC code, I would like to replace the outer lock with a mutex.
> The code only uses spinlocks, because at the time it was written, the mutex
> code didn't contain a busy wait.
> With a mutex, the code would become simpler (all the
> lock/unlock/kmalloc/relock parts could be removed).
> 
> The result would be something like:
> 
> 	mutex_lock(A)			spin_lock(B)
> 	spin_unlock_wait(B)		if (!mutex_is_locked(A))
> 	do_something()			  do_something()
> 

Should work similarly, but we'll have to audit mutex for these same
issues. I'll put it on todo.