linux-kernel - Re: [BUG] -next lockdep invalid wait context

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Message-ID: <ed93c68c-fb17-4c20-958e-0fc4ce8bcd83@paulmck-laptop>
Date: Fri, 1 Nov 2024 10:14:27 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Marco Elver <elver@...gle.com>, linux-next@...r.kernel.org,
	linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
	linux-mm@...ck.org, sfr@...b.auug.org.au, longman@...hat.com,
	boqun.feng@...il.com, cl@...ux.com, penberg@...nel.org,
	rientjes@...gle.com, iamjoonsoo.kim@....com,
	akpm@...ux-foundation.org
Subject: Re: [BUG] -next lockdep invalid wait context

On Thu, Oct 31, 2024 at 09:18:52AM +0100, Vlastimil Babka wrote:
> On 10/31/24 08:55, Sebastian Andrzej Siewior wrote:
> > On 2024-10-31 08:35:45 [+0100], Vlastimil Babka wrote:
> >> On 10/31/24 08:21, Sebastian Andrzej Siewior wrote:
> >> > On 2024-10-30 16:10:58 [-0700], Paul E. McKenney wrote:
> >> >> 
> >> >> So I need to avoid calling kfree() within an smp_call_function() handler?
> >> > 
> >> > Yes. No kmalloc()/ kfree() in IRQ context.
> >> 
> >> However, isn't this the case that the rule is actually about hardirq context
> >> on RT, and most of these operations that are in IRQ context on !RT become
> >> the threaded interrupt context on RT, so they are actually fine? Or is smp
> >> call callback a hardirq context on RT and thus it really can't do those
> >> operations?
> > 
> > interrupt handlers as of request_irq() are forced-threaded on RT so you
> > can do kmalloc()/ kfree() there. smp_call_function.*() on the other hand
> > are not threaded and invoked directly within the IRQ context.
> 
> Makes sense, thanks.
> 
> So how comes rcutorture wasn't deadlocking on RT already, is it (or RCU
> itself) doing anything differently there that avoids the kfree() from
> smp_call_function() handler?

This was scftorture rather than rcutorture.  While I know of others who
regularly run rcutorture, to the best of my knowledge I am the only one
who ever runs scftorture, which tests smp_call_function(), its friends,
and its diagnostics.

							Thanx, Paul