linux-kernel - Re: [PATCH drm-next v2 04/16] maple_tree: add flag MT_FLAGS_LOCK

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Message-ID: <Y/z4QMyIkZLi4Ruw@casper.infradead.org>
Date:   Mon, 27 Feb 2023 18:36:48 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Danilo Krummrich <dakr@...hat.com>
Cc:     matthew.brost@...el.com, dri-devel@...ts.freedesktop.org,
        corbet@....net, nouveau@...ts.freedesktop.org, ogabbay@...nel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, boris.brezillon@...labora.com,
        bskeggs@...hat.com, tzimmermann@...e.de, Liam.Howlett@...cle.com,
        bagasdotme@...il.com, christian.koenig@....com,
        jason@...kstrand.net
Subject: Re: [PATCH drm-next v2 04/16] maple_tree: add flag MT_FLAGS_LOCK_NONE

On Mon, Feb 27, 2023 at 06:39:33PM +0100, Danilo Krummrich wrote:
> On 2/21/23 19:31, Matthew Wilcox wrote:
> > Lockdep will shout at you if you get it wrong ;-)  But you can safely
> > take the spinlock before calling mas_store_gfp(GFP_KERNEL) because
> > mas_nomem() knows to drop the lock before doing a sleeping allocation.
> > Essentially you're open-coding mtree_store_range() but doing your own
> > thing in addition to the store.
> 
> As already mentioned, I went with your advice to just take the maple tree's
> internal spinlock within the GPUVA manager and leave all the other locking
> to the drivers as intended.
> 
> However, I run into the case that lockdep shouts at me for not taking the
> spinlock before calling mas_find() in the iterator macros.
> 
> Now, I definitely don't want to let the drivers take the maple tree's
> spinlock before they use the iterator macro. Of course, drivers shouldn't
> even know about the underlying maple tree of the GPUVA manager.
> 
> One way to make lockdep happy in this case seems to be taking the spinlock
> right before mas_find() and drop it right after for each iteration.

While we don't have any lockdep checking of this, you really shouldn't be
using an iterator if you're going to drop the lock between invocations.
The iterator points into the tree, so you need to invalidate the iterator
any time you drop the lock.

You don't have to use a spinlock to do a read iteration.  You can just
take the rcu_read_lock() around your iteration, as long as you can
tolerate the mild inconsistencies that RCU permits.