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Date: Wed, 7 Aug 2024 18:05:42 +0100
From: Simon Horman <horms@kernel.org>
To: Manoj Vishwanathan <manojvishy@google.com>
Cc: Alexander Lobakin <aleksander.lobakin@intel.com>,
	netdev@vger.kernel.org, David Decotigny <decot@google.com>,
	linux-kernel@vger.kernel.org,
	Tony Nguyen <anthony.l.nguyen@intel.com>,
	Przemek Kitszel <przemyslaw.kitszel@intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	intel-wired-lan@lists.osuosl.org
Subject: Re: [Intel-wired-lan] [PATCH] [PATCH iwl-net] idpf: Acquire the lock
 before accessing the xn->salt
Message-ID: <20240807170542.GE3006561@kernel.org>
References: <20240803182548.2932270-1-manojvishy@google.com>
 <20240805182159.3547482-1-manojvishy@google.com>
 <04affbd5-828a-4327-8b84-8767c1c139f1@intel.com>
 <CA+M8utN7FbwMF5QN8O0a0Qnd3ykQwq7O4QkHMVEaBj2jE9BEYw@mail.gmail.com>
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On Wed, Aug 07, 2024 at 06:58:59AM -0700, Manoj Vishwanathan wrote:
> Thanks Przemek & Olek for your quick feedback and responses.
> Hi Olek,
> I can add more details about the issue we faced in the commit message.
> The bug we had here was a virtchnl delay leading to the xn->salt
> mismatch. This could be due to several factors including default CPU
> bounded kworker workqueue for virtchnl message processing being
> starved by aggressive userspace load causing the virtchnl to be
> delayed. While debugging this issue, this locking order  appeared like
> a potential issue, hence the change was made.
> But, this change is more a clean up we felt based on concurrent access
> to the virtchnl transaction struct and does not fix the issue. This is
> more of the patch to do the right thing before we access the "xn".
> I wanted to start with a first patch to the community for acceptance
> followed by a series of other patches that are general clean up or
> improvements to IDPF in general. Will follow with with [PATCH v3]

Still, I am a little confused about the protection offered to xn->salt.

My analysis is as follows, where guarded is used loosely to mean
the lock is held.

* In idpf_vc_xn_pop_free() it is guarded by vcxn_mngr->xn_bm_lock.

* In idpf_vc_xn_exec() it is guarded by:
  1. vcxn_mngr->xn_bm_lock when idpf_vc_xn_pop_free is called
  2. idpf_vc_xn_lock, otherwise

* And with this patch, in idpf_vc_xn_forward_reply it is guarded
  by idpf_vc_xn_lock().

This doesn't seem entirely consistent.

Also, please don't top-post on Kernel mailing lists.

...