linux-kernel - Re: [PATCH v4 07/10] mmap locking API: add mmap_read_trylock_non

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Date:   Mon, 20 Apr 2020 12:23:22 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Michel Lespinasse <walken@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Liam Howlett <Liam.Howlett@...cle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v4 07/10] mmap locking API: add
 mmap_read_trylock_non_owner()

On Mon, Apr 20, 2020 at 02:22:11PM -0400, Daniel Jordan wrote:
> On Tue, Apr 14, 2020 at 05:43:50PM -0700, Michel Lespinasse wrote:
> > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> > index 11d41f0c7005..998968659892 100644
> > --- a/kernel/bpf/stackmap.c
> > +++ b/kernel/bpf/stackmap.c
> > @@ -317,7 +316,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> >  	 * with build_id.
> >  	 */
> >  	if (!user || !current || !current->mm || irq_work_busy ||
> > -	    mmap_read_trylock(current->mm) == 0) {
> > +	    !mmap_read_trylock_non_owner(current->mm)) {
> >  		/* cannot access current->mm, fall back to ips */
> >  		for (i = 0; i < trace_nr; i++) {
> >  			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> > @@ -342,16 +341,10 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> >  	}
> >  
> >  	if (!work) {
> > -		mmap_read_unlock(current->mm);
> > +		mmap_read_unlock_non_owner(current->mm);
> 
> These 'non_owner' calls are not intuitive because current _is the owner, so the
> v3 version seems better, even if it adds a special wrapper for rwsem_release.
> 
> Though it makes some sense if you think, "we're consistently using the
> non_owner APIs because there's a legitimate use somewhere else," so I'm fine
> either way.

I'm not really a big fan of v3 nor v4.  What I'd like to see is a
'transfer of ownership' API.  This could be to a different task, IRQ work,
RCU, softirq, timer, ...

That would let us track locking dependencies across complex flows, eg this
wouldn't be warned about right now:

rcu_work():
	lock(C)
	kfree(B)
	unlock(A)
	unlock(C)

thread 1:
	lock(A)
	call_rcu(B)

thread 2:
	lock(C)
	synchronize_rcu()
	unlock(C)

but if we had an API that transferred ownership of A to RCU, then we'd
see the C->RCU->A->C cycle.

This is perhaps a bit much work to require of Laurent in order to get
this patchset merged, but something to think about.