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

["Paul E. McKenney" <paulmck@kernel.org>]

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.

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
>