linux-kernel - Re: [btrfs/rt] lockdep false positive

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Date:   Mon, 23 Jan 2017 10:07:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Galbraith <umgwanakikbuti@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [btrfs/rt] lockdep false positive

On Sun, Jan 22, 2017 at 06:45:14PM +0100, Mike Galbraith wrote:
> On Sun, 2017-01-22 at 09:46 +0100, Mike Galbraith wrote:
> > Greetings btrfs/lockdep wizards,
> > 
> > RT trees have trouble with the BTRFS lockdep positive avoidance lock
> > class dance (see disk-io.c).  Seems the trouble is due to RT not having
> > a means of telling lockdep that its rwlocks are recursive for read by
> > the lock owner only, combined with the BTRFS lock class dance assuming
> > that read_lock() is annotated rwlock_acquire_read(), which RT cannot
> > do, as that would be a big fat lie.
> > 
> > Creating a rt_read_lock_shared() for btrfs_clear_lock_blocking_rw() did
> > indeed make lockdep happy as a clam for test purposes.  (hm, submitting
> > that would be excellent way to replenish frozen shark supply:)
> > 
> > Ideas?

Not having looked at anything much, currently lockdep does not in fact
model rwlock properly as is.

Note that rwlock is _not_ in fact reader biased like it used to be, that
is, read_lock() will block if there is a pending writer (just like
rwsem), with the exception when read_lock() happend in_interrupt()
(because tasklist_lock).