linux-kernel - seq_lock and lockdep_is

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Date:   Fri, 21 Feb 2020 18:36:26 +0100
From:   Jann Horn <jannh@...gle.com>
To:     David Howells <dhowells@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: seq_lock and lockdep_is_held() assertions

adding some locking folks to the thread...

On Fri, Feb 21, 2020 at 6:06 PM David Howells <dhowells@...hat.com> wrote:
> Jann Horn <jannh@...gle.com> wrote:
> > On Fri, Feb 21, 2020 at 1:24 PM David Howells <dhowells@...hat.com> wrote:
> > > What's the best way to write a lockdep assertion?
> > >
> > >         BUG_ON(!lockdep_is_held(lock));
> >
> > lockdep_assert_held(lock) is the normal way, I think - that will
> > WARN() if lockdep is enabled and the lock is not held.
>
> Okay.  But what's the best way with a seqlock_t?  It has two dep maps in it.
> Do I just ignore the one attached to the spinlock?

Uuuh... very good question. Looking at how the seqlock_t helpers use
the dep map of the seqlock, I don't think lockdep asserts work for
asserting that you're in the read side of a seqlock?

read_seqbegin_or_lock() -> read_seqbegin() -> read_seqcount_begin() ->
seqcount_lockdep_reader_access() does seqcount_acquire_read() (which
maps to lock_acquire_shared_recursive()), but immediately following
that calls seqcount_release() (which maps to lock_release())?

So I think lockdep won't consider you to be holding any locks after
read_seqbegin_or_lock() if the lock wasn't taken?