linux-kernel - Re: [PATCH v9 3/9] PCI: Create PCI library functions in support of DOE mailboxes.

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Date:   Tue, 31 May 2022 10:56:52 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     "ira.weiny@...el.com" <ira.weiny@...el.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ben Widawsky <ben@...dawsk.net>, linux-kernel@...r.kernel.org,
        linux-cxl@...r.kernel.org, linux-pci@...r.kernel.org,
        a.manzanares@...sung.com
Subject: Re: [PATCH v9 3/9] PCI: Create PCI library functions in support of
 DOE mailboxes.

On Tue, 31 May 2022, Davidlohr Bueso wrote:

>On Tue, 31 May 2022, ira.weiny@...el.com wrote:
>
>>+static void doe_statemachine_work(struct work_struct *work)
>>+{
>>+	struct delayed_work *w = to_delayed_work(work);
>>+	struct pci_doe_mb *doe_mb = container_of(w, struct pci_doe_mb,
>>+						 statemachine);
>>+	struct pci_dev *pdev = doe_mb->pdev;
>>+	int offset = doe_mb->cap_offset;
>>+	struct pci_doe_task *task;
>>+	u32 val;
>>+	int rc;
>>+
>>+	mutex_lock(&doe_mb->task_lock);
>>+	task = doe_mb->cur_task;
>>+	mutex_unlock(&doe_mb->task_lock);
>
>Instead of a mutex, would it be better to use a rwsem here to protect
>the state machine and allow for concurrent reads for the work callback?
>It is a general interface and a trivial change, but not sure how much
>performance is cared about.

Actually why is this a sleeping lock at all? Afaict all critical regions
are short and just deal with loads and stores of oe_mb->task_lock (and
pci_doe_submit_task also checks the doe_mb->flags with the lock held).
This could be a spinlock or similarly a rwlock.

Thanks,
Davidlohr