RE: [v3 PATCH] rhashtable: Fix rhashtable_try_insert test

[Michael Kelley]

From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Sent: Monday, January 13, 2025 7:15 PM
> 
> On Fri, Jan 10, 2025 at 06:22:40PM +0000, Michael Kelley wrote:
> >
> > This patch passes my tests. I'm doing a narrow test to verify that
> > the boot failure when opening the Mellanox NIC is no longer occurring.
> > I also unloaded/reloaded the mlx5 driver a couple of times. For good
> > measure, I then did a full Linux kernel build, and all is good. My testing
> > does not broadly verify correct operation of rhashtable except as it
> > gets exercised implicitly by these basic tests.
> 
> Thanks for testing! The patch needs one more change though as
> moving the atomic_inc outside of the lock was a bad idea on my
> part.  This could cause atomic_inc/atomic_dec to be reordered
> thus resulting in an underflow.
> 
> Thanks,

I've tested this version of the fix in the same limited way as
before. All is good. I'm still testing against linux-next20250108
from a few days ago, but that should not make any difference.
I have *not* reviewed the code change itself.

Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> 
> ---8<---
> The test on whether rhashtable_insert_one did an insertion relies
> on the value returned by rhashtable_lookup_one.  Unfortunately that
> value is overwritten after rhashtable_insert_one returns.  Fix this
> by moving the test before data gets overwritten.
> 
> Simplify the test as only data == NULL matters.
> 
> Finally move atomic_inc back within the lock as otherwise it may
> be reordered with the atomic_dec on the removal side, potentially
> leading to an underflow.
> 
> Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Fixes: e1d3422c95f0 ("rhashtable: Fix potential deadlock by moving schedule_work outside lock")
> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> 
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index bf956b85455a..0e9a1d4cf89b 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -611,21 +611,23 @@ static void *rhashtable_try_insert(struct rhashtable *ht,
> const void *key,
>  			new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
>  			data = ERR_PTR(-EAGAIN);
>  		} else {
> +			bool inserted;
> +
>  			flags = rht_lock(tbl, bkt);
>  			data = rhashtable_lookup_one(ht, bkt, tbl,
>  						     hash, key, obj);
>  			new_tbl = rhashtable_insert_one(ht, bkt, tbl,
>  							hash, obj, data);
> +			inserted = data && !new_tbl;
> +			if (inserted)
> +				atomic_inc(&ht->nelems);
>  			if (PTR_ERR(new_tbl) != -EEXIST)
>  				data = ERR_CAST(new_tbl);
> 
>  			rht_unlock(tbl, bkt, flags);
> 
> -			if (PTR_ERR(data) == -ENOENT && !new_tbl) {
> -				atomic_inc(&ht->nelems);
> -				if (rht_grow_above_75(ht, tbl))
> -					schedule_work(&ht->run_work);
> -			}
> +			if (inserted && rht_grow_above_75(ht, tbl))
> +				schedule_work(&ht->run_work);
>  		}
>  	} while (!IS_ERR_OR_NULL(new_tbl));
>