netdev - Re: [PATCH net] s390/ism: Fix locking for forwarding of IRQs and events to clients

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Date: Thu, 06 Jul 2023 14:00:51 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Paolo Abeni <pabeni@...hat.com>, Alexandra Winter
 <wintera@...ux.ibm.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Heiko
 Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander
 Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger
 <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Jan
 Karcher <jaka@...ux.ibm.com>,
        Stefan Raspl <raspl@...ux.ibm.com>,
        "David S.
 Miller" <davem@...emloft.net>
Cc: Julian Ruess <julianr@...ux.ibm.com>, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] s390/ism: Fix locking for forwarding of IRQs and
 events to clients

On Thu, 2023-07-06 at 12:47 +0200, Paolo Abeni wrote:
> On Wed, 2023-07-05 at 14:17 +0200, Niklas Schnelle wrote:
> > The clients array references all registered clients and is protected by
> > the clients_lock. Besides its use as general list of clients the clients
> > array is accessed in ism_handle_irq() to forward IRQs and events to
> > clients. This use in an interrupt handler thus requires all code that
> > takes the clients_lock to be IRQ save.
> > 
> > This is problematic since the add() and remove() callbacks which are
> > called for all clients when an ISM device is added or removed cannot be
> > called directly while iterating over the clients array and holding the
> > clients_lock since clients need to allocate and/or take mutexes in these
> > callbacks. To deal with this the calls get pushed to workqueues with
> > additional housekeeping to be able to wait for the completion outside
> > the clients_lock.
> > 
> > Moreover while the clients_lock is taken in the IRQ handler when calling
> > handle_event() it is incorrectly not held during the
> > client->handle_irq() call and for the preceding clients[] access. This
> > leaves the clients array unprotected. Similarly the accesses to
> > ism->sba_client_arr[] in ism_register_dmb() and ism_unregister_dmb() are
> > also not protected by any lock. This is especially problematic as the
> > the client ID from the ism->sba_client_arr[] is not checked against
> > NO_CLIENT.
> > 
> > Instead of expanding the use of the clients_lock further add a separate
> > array in struct ism_dev which references clients subscribed to the
> > device's events and IRQs. This array is protected by ism->lock which is
> > already taken in ism_handle_irq() and can be taken outside the IRQ
> > handler when adding/removing subscribers or the accessing
> > ism->sba_client_arr[].
> > 
> > With the clients_lock no longer accessed from IRQ context it is turned
> > into a mutex and the add and remove workqueues plus their housekeeping
> > can be removed in favor of simple direct calls.
> > 
> > Fixes: 89e7d2ba61b7 ("net/ism: Add new API for client registration")
> > Tested-by: Julian Ruess <julianr@...ux.ibm.com>
> > Reviewed-by: Julian Ruess <julianr@...ux.ibm.com>
> > Reviewed-by: Alexandra Winter <wintera@...ux.ibm.com>
> > Reviewed-by: Wenjia Zhang <wenjia@...ux.ibm.com>
> > Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> > ---
> > Note: I realize this is a rather large patch. So I'd understand if it's not
> > acceptable as is and needs to be broken up. That said it removes more lines
> > than it adds and the complexity of the resulting code is in my opinion reduced.
> 
> This is indeed unusually large for a -net patch. IMHO it would be
> better split it in 2 separated patches: 1 introducing the ism->lock and
> one turning the clients_lock in a mutex. The series should still target
> -net, but should be more easily reviewable.
> 
> Thanks,
> 
> Paolo
> 

Sounds reasonable. Patch 1 would introduce and use the ism->subs[]
array under the ism->lock and also protect the ism->sba_client_arr[]
under that lock. Patch 2 would then turn clients_lock into a mutex and
remove the workqueues. I think strictly speaking the second one then
isn't a fix but let's see. @Alexandra, Wenjia, Julian I'll drop your R-
bs as its a larger rework but I hope to end up at the same code after
both patches so should be easy to re-revievie for you.

Thanks,
Niklas