From: "Tobin C. Harding" <me@tobin.cc>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Subject: Re: dcache locking question
Date: Fri, 15 Mar 2019 12:38:05 +1100 [thread overview]
Message-ID: <20190315013805.GA23541@eros.localdomain> (raw)
In-Reply-To: <20190314230904.GO19508@bombadil.infradead.org>
On Thu, Mar 14, 2019 at 04:09:05PM -0700, Matthew Wilcox wrote:
> On Fri, Mar 15, 2019 at 09:56:32AM +1100, Tobin C. Harding wrote:
> > Could someone please explain to me how the locking order commented at
> > the top of the file is not violated in the following:
> >
> > >From top of fs/dcache.c
> >
> > * If there is an ancestor relationship:
> > * dentry->d_parent->...->d_parent->d_lock
> > * ...
> > * dentry->d_parent->d_lock
> > * dentry->d_lock
> >
> >
> > dentry_kill() appears to require caller to hold the dentry->d_lock yet
> > it locks the parent with spin_trylock(&parent->d_lock), if this
> > fails it calls __lock_parent() which releases the dentry->d_lock before
> > locking the parent and re-acquiring [1] the dentry->d_lock . Is this not
> > locking in two different orders?
>
> What you're describing here is how we work around having to lock in the
> wrong order. There are "a few" places in the kernel where we do this.
> Calling spin_trylock() won't deadlock -- it'll just return failure.
> At that point, we drop the child, spin waiting for the parent, then lock
> the child.
>
> > [1] I do not fully understand the spin_lock_nested() macro.
>
> It describes to lockdep that, while it looks like we're acquiring a lock
> that we already own, there's actually a hierarchy of locks that are in
> the same lock class; we're attempting to acquire a lock in "subclass N".
> N is allowed to be between 0-7, inclusive.
ok, cool. Thanks for taking the time to explain this.
Tobin
next prev parent reply other threads:[~2019-03-15 1:38 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-14 22:56 dcache locking question Tobin C. Harding
2019-03-14 23:09 ` Matthew Wilcox
2019-03-15 1:38 ` Tobin C. Harding [this message]
2019-03-14 23:19 ` Tobin C. Harding
2019-03-15 1:50 ` Al Viro
2019-03-15 17:38 ` Eric Biggers
2019-03-15 18:54 ` Al Viro
2019-03-16 22:31 ` Paul E. McKenney
2019-03-17 0:18 ` Al Viro
2019-03-17 0:50 ` Paul E. McKenney
2019-03-17 2:20 ` James Bottomley
2019-03-17 3:06 ` Al Viro
2019-03-17 4:23 ` James Bottomley
2019-03-18 0:35 ` Paul E. McKenney
2019-03-18 16:26 ` James Bottomley
2019-03-18 17:11 ` Paul E. McKenney
2019-03-19 15:45 ` Paul E. McKenney
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