[v2] qat: fix deadlock in backlog processing

Hi


On Fri, 22 Sep 2023, Giovanni Cabiddu wrote:

> Hi Mikulas,
> 
> many thanks for reporting this issue and finding a solution.
> 
> On Thu, Sep 21, 2023 at 10:53:55PM +0200, Mikulas Patocka wrote:
> > I was evaluating whether it is feasible to use QAT with dm-crypt (the 
> > answer is that it is not - QAT is slower than AES-NI for this type of 
> > workload; QAT starts to be benefical for encryption requests longer than 
> > 64k).
> Correct. Is there anything that we can do to batch requests in a single
> call?

Ask Herbert Xu. I think it would complicate the design of crypto API.

> Sometime ago there was some work done to build a geniv template cipher
> and optimize dm-crypt to encrypt larger block sizes in a single call,
> see [1][2]. Don't know if that work was completed.
>
> >And I got some deadlocks.
> Ouch!
> 
> > The reason for the deadlocks is this: suppose that one of the "if"
> > conditions in "qat_alg_send_message_maybacklog" is true and we jump to the
> > "enqueue" label. At this point, an interrupt comes in and clears all
> > pending messages. Now, the interrupt returns, we grab backlog->lock, add
> > the message to the backlog, drop backlog->lock - and there is no one to
> > remove the backlogged message out of the list and submit it.
> Makes sense. In my testing I wasn't able to reproduce this condition.

I reproduced it with this:
Use a system with two Intel(R) Xeon(R) Gold 5420+ processors
Use a kernel 6.6-rc2
Patch the kernel, so that dm-crypt uses QAT - that is, in 
	drivers/md/dm-crypt.c, replace all strings 
	"CRYPTO_ALG_ALLOCATES_MEMORY" with "0"
Use .config from RHEL-9.4 beta and compile the kernel
On the system, disable hyperthreading with
	"echo off >/sys/devices/system/cpu/smt/control"
Activate dm-crypt on the top of nvme:
	"cryptsetup create cr /dev/nvme3n1 --sector-size=4096"
Run fio in a loop:
	"while true; do
		fio --ioengine=psync --iodepth=1 --rw=randwrite --direct=1 
		--end_fsync=1 --bs=64k --numjobs=56 --time_based 
		--runtime=10 --group_reporting --name=job 
		--filename=/dev/mapper/cr
	done"

With this setup, I get a deadlock in a few iterations of fio.

> > I fixed it with this patch - with this patch, the test passes and there
> > are no longer any deadlocks. I didn't want to add a spinlock to the hot
> > path, so I take it only if some of the condition suggests that queuing may
> > be required.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > Cc: stable@vger.kernel.org
> The commit message requires a bit of rework to describe the change.

I improved the message and I send a second version of the patch.

> Also, deserves a fixes tag.

"Fixes" tag is for something that worked and that was broken in some 
previous commit. A quick search through git shows that QAT backlogging was 
broken since the introduction of QAT.

> > 
> > ---
> >  drivers/crypto/intel/qat/qat_common/qat_algs_send.c |   31 ++++++++++++--------
> >  1 file changed, 20 insertions(+), 11 deletions(-)
> > 
> > Index: linux-2.6/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> > ===================================================================
> > --- linux-2.6.orig/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> > +++ linux-2.6/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> > @@ -40,16 +40,6 @@ void qat_alg_send_backlog(struct qat_ins
> >  	spin_unlock_bh(&backlog->lock);
> >  }
> >  
> > -static void qat_alg_backlog_req(struct qat_alg_req *req,
> > -				struct qat_instance_backlog *backlog)
> > -{
> > -	INIT_LIST_HEAD(&req->list);
> Is the initialization of an element no longer needed?

It was never needed. list_add_tail calls __list_add and __list_add 
overwrites new->next and new->prev without reading them. So, there's no 
need to initialize them.

> > -
> > -	spin_lock_bh(&backlog->lock);
> > -	list_add_tail(&req->list, &backlog->list);
> > -	spin_unlock_bh(&backlog->lock);
> > -}
> > -
> >  static int qat_alg_send_message_maybacklog(struct qat_alg_req *req)
> >  {
> >  	struct qat_instance_backlog *backlog = req->backlog;
> > @@ -71,8 +61,27 @@ static int qat_alg_send_message_maybackl
> >  	return -EINPROGRESS;
> >  
> >  enqueue:
> > -	qat_alg_backlog_req(req, backlog);
> > +	spin_lock_bh(&backlog->lock);
> > +
> > +	/* If any request is already backlogged, then add to backlog list */
> > +	if (!list_empty(&backlog->list))
> > +		goto enqueue2;
> >  
> > +	/* If ring is nearly full, then add to backlog list */
> > +	if (adf_ring_nearly_full(tx_ring))
> > +		goto enqueue2;
> > +
> > +	/* If adding request to HW ring fails, then add to backlog list */
> > +	if (adf_send_message(tx_ring, fw_req))
> > +		goto enqueue2;
> In a nutshell, you are re-doing the same steps taking the backlog lock.
> 
> It should be possible to re-write it so that there is a function that
> attempts enqueuing and if it fails, then the same is called again taking
> the lock.
> If you want I can rework it and resubmit.

Yes, if you prefer it this way, I reworked the patch so that we execute 
the same code with or without the spinlock held.

> > +
> > +	spin_unlock_bh(&backlog->lock);
> > +	return -EINPROGRESS;
> > +
> > +enqueue2:
> > +	list_add_tail(&req->list, &backlog->list);
> > +
> > +	spin_unlock_bh(&backlog->lock);
> >  	return -EBUSY;
> >  }
> 
> [1] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1276510.html
> [2] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1428293.html
> 
> Regards,
> 
> -- 
> Giovanni
> 

From: Mikulas Patocka <mpatocka@redhat.com>
Subject: [PATCH] qat: fix deadlock in backlog processing

I was testing QAT with dm-crypt and I got some deadlocks.

The reason for the deadlocks is this: suppose that one of the "if"
conditions in "qat_alg_send_message_maybacklog" is true and we jump to the
"enqueue" label. At this point, an interrupt comes in and clears all
pending messages. Now, the interrupt returns, we grab backlog->lock, add
the message to the backlog, drop backlog->lock - and there is no one to
remove the backlogged message out of the list and submit it.

In order to fix the bug, we must hold the spinlock backlog->lock when we 
perform test for free space in the ring - so that the test for free space 
and adding the request to a backlog is atomic and can't be interrupted by 
an interrupt. Every completion interrupt calls qat_alg_send_backlog which 
grabs backlog->lock, so holding this spinlock is sufficient to synchronize 
with interrupts.

I didn't want to add a spinlock unconditionally to the hot path for 
performance reasons, so I take it only if some of the condition suggests 
that queuing may be required.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

---
 drivers/crypto/intel/qat/qat_common/qat_algs_send.c |   23 ++++++++++----------
 1 file changed, 12 insertions(+), 11 deletions(-)

On Fri, 29 Sep 2023, Giovanni Cabiddu wrote:

> On Fri, Sep 22, 2023 at 06:34:16PM +0200, Mikulas Patocka wrote:
> >
> > > Also, deserves a fixes tag.
> > 
> > "Fixes" tag is for something that worked and that was broken in some 
> > previous commit.
> That's right.
> 
> > A quick search through git shows that QAT backlogging was 
> > broken since the introduction of QAT.
> The driver was moved from drivers/crypto/qat to drivers/crypto/intel/qat
> that's why you see a single patch.
> This fixes 386823839732 ("crypto: qat - add backlog mechanism")

But before 386823839732 it also didn't work - it returned -EBUSY without 
queuing the request and deadlocked.

> Thanks - when I proposed the rework I was thinking at a solution without
> gotos. Here is a draft:

Yes - it is possible to fix it this way.



> ------------8<----------------
>  .../intel/qat/qat_common/qat_algs_send.c      | 40 ++++++++++---------
>  1 file changed, 22 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/crypto/intel/qat/qat_common/qat_algs_send.c b/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> index bb80455b3e81..18c6a233ab96 100644
> --- a/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> +++ b/drivers/crypto/intel/qat/qat_common/qat_algs_send.c
> @@ -40,17 +40,7 @@ void qat_alg_send_backlog(struct qat_instance_backlog *backlog)
>  	spin_unlock_bh(&backlog->lock);
>  }
>  
> -static void qat_alg_backlog_req(struct qat_alg_req *req,
> -				struct qat_instance_backlog *backlog)
> -{
> -	INIT_LIST_HEAD(&req->list);
> -
> -	spin_lock_bh(&backlog->lock);
> -	list_add_tail(&req->list, &backlog->list);
> -	spin_unlock_bh(&backlog->lock);
> -}
> -
> -static int qat_alg_send_message_maybacklog(struct qat_alg_req *req)
> +static bool qat_alg_try_enqueue(struct qat_alg_req *req)
>  {
>  	struct qat_instance_backlog *backlog = req->backlog;
>  	struct adf_etr_ring_data *tx_ring = req->tx_ring;
> @@ -58,22 +48,36 @@ static int qat_alg_send_message_maybacklog(struct qat_alg_req *req)
>  
>  	/* If any request is already backlogged, then add to backlog list */
>  	if (!list_empty(&backlog->list))
> -		goto enqueue;
> +		return false;
>  
>  	/* If ring is nearly full, then add to backlog list */
>  	if (adf_ring_nearly_full(tx_ring))
> -		goto enqueue;
> +		return false;
>  
>  	/* If adding request to HW ring fails, then add to backlog list */
>  	if (adf_send_message(tx_ring, fw_req))
> -		goto enqueue;
> +		return false;
>  
> -	return -EINPROGRESS;
> +	return true;
> +}
>  
> -enqueue:
> -	qat_alg_backlog_req(req, backlog);
>  
> -	return -EBUSY;
> +static int qat_alg_send_message_maybacklog(struct qat_alg_req *req)
> +{
> +	struct qat_instance_backlog *backlog = req->backlog;
> +	int ret = -EINPROGRESS;
> +
> +	if (qat_alg_try_enqueue(req))
> +		return ret;
> +
> +	spin_lock_bh(&backlog->lock);
> +	if (!qat_alg_try_enqueue(req)) {
> +		list_add_tail(&req->list, &backlog->list);
> +		ret = -EBUSY;
> +	}
> +	spin_unlock_bh(&backlog->lock);
> +
> +	return ret;
>  }
>  
>  int qat_alg_send_message(struct qat_alg_req *req)
> -- 
> 2.41.0


Reviwed-by: Mikulas Patocka <mpatocka@redhat.com>

Mikulas