Re: [PATCH] scftorture: Use workqueue to free scf_check

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Nov 01, 2024 at 04:35:28PM -0700, Paul E. McKenney wrote:
> On Fri, Nov 01, 2024 at 12:54:38PM -0700, Boqun Feng wrote:
> > Paul reported an invalid wait context issue in scftorture catched by
> > lockdep, and the cause of the issue is because scf_handler() may call
> > kfree() to free the struct scf_check:
> > 
> > 	static void scf_handler(void *scfc_in)
> >         {
> >         [...]
> >                 } else {
> >                         kfree(scfcp);
> >                 }
> >         }
> > 
> > (call chain anlysis from Marco Elver)
> > 
> > This is problematic because smp_call_function() uses non-threaded
> > interrupt and kfree() may acquire a local_lock which is a sleepable lock
> > on RT.
> > 
> > The general rule is: do not alloc or free memory in non-threaded
> > interrupt conntexts.
> > 
> > A quick fix is to use workqueue to defer the kfree(). However, this is
> > OK only because scftorture is test code. In general the users of
> > interrupts should avoid giving interrupt handlers the ownership of
> > objects, that is, users should handle the lifetime of objects outside
> > and interrupt handlers should only hold references to objects.
> > 
> > Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
> > Link: https://lore.kernel.org/lkml/41619255-cdc2-4573-a360-7794fc3614f7@paulmck-laptop/
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> 
> Thank you!
> 
> I was worried that putting each kfree() into a separate workqueue handler
> would result in freeing not keeping up with allocation for asynchronous
> testing (for example, scftorture.weight_single=1), but it seems to be
> doing fine in early testing.
> 

I shared the same worry, so it's why I added the comments before
queue_work() saying it's only OK because it's test code, it's certainly
not something recommended for general use.

But glad it turns out OK so far for scftorture ;-)

Regards,
Boqun

> So I have queued this in my -rcu tree for review and further testing.
> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/scftorture.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/scftorture.c b/kernel/scftorture.c
> > index 44e83a646264..ab6dcc7c0116 100644
> > --- a/kernel/scftorture.c
> > +++ b/kernel/scftorture.c
> > @@ -127,6 +127,7 @@ static unsigned long scf_sel_totweight;
> >  
> >  // Communicate between caller and handler.
> >  struct scf_check {
> > +	struct work_struct work;
> >  	bool scfc_in;
> >  	bool scfc_out;
> >  	int scfc_cpu; // -1 for not _single().
> > @@ -252,6 +253,13 @@ static struct scf_selector *scf_sel_rand(struct torture_random_state *trsp)
> >  	return &scf_sel_array[0];
> >  }
> >  
> > +static void kfree_scf_check_work(struct work_struct *w)
> > +{
> > +	struct scf_check *scfcp = container_of(w, struct scf_check, work);
> > +
> > +	kfree(scfcp);
> > +}
> > +
> >  // Update statistics and occasionally burn up mass quantities of CPU time,
> >  // if told to do so via scftorture.longwait.  Otherwise, occasionally burn
> >  // a little bit.
> > @@ -296,7 +304,10 @@ static void scf_handler(void *scfc_in)
> >  		if (scfcp->scfc_rpc)
> >  			complete(&scfcp->scfc_completion);
> >  	} else {
> > -		kfree(scfcp);
> > +		// Cannot call kfree() directly, pass it to workqueue. It's OK
> > +		// only because this is test code, avoid this in real world
> > +		// usage.
> > +		queue_work(system_wq, &scfcp->work);
> >  	}
> >  }
> >  
> > @@ -335,6 +346,7 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
> >  			scfcp->scfc_wait = scfsp->scfs_wait;
> >  			scfcp->scfc_out = false;
> >  			scfcp->scfc_rpc = false;
> > +			INIT_WORK(&scfcp->work, kfree_scf_check_work);
> >  		}
> >  	}
> >  	switch (scfsp->scfs_prim) {
> > -- 
> > 2.45.2
> >