linux-kernel - Re: d1fc031747 ("sched/wait: assert the wait_queue_head lock is .."): EIP: __wake_up

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Date:   Thu, 14 Dec 2017 14:10:37 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kernel test robot <fengguang.wu@...el.com>,
        Christoph Hellwig <hch@....de>, LKP <lkp@...org>,
        linux-kernel@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        wfg@...ux.intel.com, Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: d1fc031747 ("sched/wait: assert the wait_queue_head lock is
        .."):  EIP: __wake_up_common

On Thu, Dec 14, 2017 at 04:58:09AM -0800, Matthew Wilcox wrote:
> On Wed, Dec 13, 2017 at 05:03:00PM -0800, Andrew Morton wrote:
> > >     sched/wait: assert the wait_queue_head lock is held in __wake_up_common
> > >     
> > >     Better ensure we actually hold the lock using lockdep than just commenting
> > >     on it.  Due to the various exported _locked interfaces it is far too easy
> > >     to get the locking wrong.
> > 
> > I'm probably sitting on an older version.  I've dropped
> > 
> > epoll: use the waitqueue lock to protect ep->wq
> > sched/wait: assert the wait_queue_head lock is held in __wake_up_common
> 
> Looks pretty clear to me that userfaultfd is also abusing the wake_up_locked
> interfaces:
> 
>         spin_lock(&ctx->fault_pending_wqh.lock);
>         __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
>         __wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
>         spin_unlock(&ctx->fault_pending_wqh.lock);
> 
> Sure, it's locked, but not by the lock you thought it was going to be.
> 
> There doesn't actually appear to be a bug here; fault_wqh is always serialised
> by fault_pending_wqh.lock, but lockdep can't know that.  I think this patch
> will solve the problem.

Or userfaultfd could just always use the waitqueue lock, similar to what
we are doing in epoll.

But unless someone care about micro-optimizatations I'm tempted to
add your patch to the next iteration of the series.