Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()

From: Peter Zijlstra <peterz@infradead.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>,
	Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Chandan Babu R <chandan.babu@oracle.com>,
	"Darrick J . Wong" <djwong@kernel.org>,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()
Date: Tue, 12 Sep 2023 16:02:28 +0200	[thread overview]
Message-ID: <20230912140228.GA22248@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20230912135213.GA22127@noisy.programming.kicks-ass.net>

On Tue, Sep 12, 2023 at 03:52:13PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 12, 2023 at 01:28:13PM +0100, Matthew Wilcox wrote:
> > On Tue, Sep 12, 2023 at 11:03:42AM +0200, Peter Zijlstra wrote:
> > > If not, then sure we can do this; it's not like I managed to get rid of
> > > muteX_is_locked() -- and I actually tried at some point :/
> > > 
> > > And just now I grepped for it, and look what I find:
> > > 
> > > drivers/hid/hid-nintendo.c:     if (unlikely(mutex_is_locked(&ctlr->output_mutex))) {
> > > drivers/nvdimm/btt.c:           if (mutex_is_locked(&arena->err_lock)
> > > 
> > > And there's more :-(
> > 
> > Are these actually abuse?  I looked at these two, and they both seem to
> > be asking "Does somebody else currently have this mutex?" rather than
> > "Do I have this mutex?".
> 
> It's effectively a random number generator in that capacity. Someone
> might have it or might have had it when you looked and no longer have
> it, or might have it now but not when you asked.

Also, there's more fun; the 'is_locked' store from spin_lock() (or
mutex, or whatever) is not ordered vs any other write inside the
critical section.

So something like:

	bar = 0;

	CPU0			CPU1

	spin_lock(&foo)		
	bar = 1;		x = READ_ONCE(bar)
				y = spin_is_locked(&foo);
	spin_unlock(&foo);

can have x==1 && y==0, even though CPU0 is currently inside the critical
section.

Normally that doesn't matter, and for the program-order case where you
ask 'am I holding the lock' this obviously cannot go wrong. But the
moment you ask: 'is someone else holding the lock' it all goes sideways
real fast.

We've been there, done that, got a t-shirt etc..