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Date: Thu, 24 May 2018 23:14:05 +0200
From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        psodagud@codeaurora.org, Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>, Will Drewry <wad@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Eric Biggers <ebiggers@google.com>,
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        Vegard Nossum <vegard.nossum@oracle.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        paulmck@linux.vnet.ibm.com, stern@rowland.harvard.edu
Subject: Re: write_lock_irq(&tasklist_lock)
Message-ID: <20180524211405.GA7206@andrea>
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> Yeah, lemme put some details here:
> 
> So we have three cases:
> 
> Case #1 (from Will)
> 
> 	P0:			P1:			P2:
> 
> 	spin_lock(&slock)	read_lock(&rwlock)
> 							write_lock(&rwlock)
> 	read_lock(&rwlock)	spin_lock(&slock)
> 
> , which is a deadlock, and couldn't not be detected by lockdep yet. And
> lockdep could detect this with the patch I attach at the end of the
> mail.
> 
> Case #2
> 
> 	P0:			P1:			P2:
> 
> 	<in irq handler>
> 	spin_lock(&slock)	read_lock(&rwlock)
> 							write_lock(&rwlock)
> 	read_lock(&rwlock)	spin_lock_irq(&slock)
> 
> , which is not a deadlock, as the read_lock() on P0 can use the unfair
> fastpass.
> 
> Case #3
> 
> 	P0:			P1:			P2:
> 
> 				<in irq handler>
> 	spin_lock_irq(&slock)	read_lock(&rwlock)
> 							write_lock_irq(&rwlock)
> 	read_lock(&rwlock)	spin_lock(&slock)
> 
> , which is a deadlock, as the read_lock() on P0 cannot use the fastpass.

Mmh, I'm starting to think that, maybe, we need a model (a tool) to
distinguish these and other(?) cases (sorry, I could not resist ;-)

[...]


> ------------------->8
> Subject: [PATCH] locking: More accurate annotations for read_lock()
> 
> On the archs using QUEUED_RWLOCKS, read_lock() is not always a recursive
> read lock, actually it's only recursive if in_interrupt() is true. So

Mmh, taking the "common denominator" over archs/Kconfig options and
CPU states, this would suggest that read_lock() is non-recursive;

it looks like I can say "good-bye" to my idea to define (formalize)
consistent executions/the memory ordering of RW-LOCKS "by following"
the following _emulation_:

void read_lock(rwlock_t *s)
{
	r0 = atomic_fetch_inc_acquire(&s->val);
}

void read_unlock(rwlock_t *s)
{
	r0 = atomic_fetch_sub_release(&s->val);
}

void write_lock(rwlock_t *s)
{
	r0 = atomic_cmpxchg_acquire(&s->val, 0, -1);
}

void write_unlock(rwlock_t *s)
{
	atomic_set_release(&s->val, 0);
}

filter (~read_lock:r0=-1 /\ write_lock:r0=0)

[...]


> The code is done, I'm just working on the rework for documention stuff,
> so if anyone is interested, could try it out ;-)

Any idea on how to "educate" the LKMM about this code/documentation?

  Andrea