linux-kernel - Re: [PATCH v3 2/6] cxl/mbox: Add GET_POISON

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Message-ID: <Y5FeVySReYXU6E2p@aschofie-mobl2>
Date:   Wed, 7 Dec 2022 19:47:35 -0800
From:   Alison Schofield <alison.schofield@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-cxl@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] cxl/mbox: Add GET_POISON_LIST mailbox command

On Wed, Dec 07, 2022 at 01:39:58PM -0800, Dan Williams wrote:
> Alison Schofield wrote:
> > On Tue, Dec 06, 2022 at 06:41:34PM -0800, Dan Williams wrote:
> > > alison.schofield@ wrote:
> [..]
> > > > +int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
> > > > +		       struct cxl_region *cxlr)
> > > > +{
> > > > +	struct cxl_dev_state *cxlds = cxlmd->cxlds;
> > > > +	const char *memdev_name = dev_name(&cxlmd->dev);
> > > > +	const char *pcidev_name = dev_name(cxlds->dev);
> > > > +	struct cxl_mbox_poison_payload_out *po;
> > > > +	struct cxl_mbox_poison_payload_in pi;
> > > > +	int nr_records = 0;
> > > > +	int rc;
> > > > +
> > > > +	po = kvmalloc(cxlds->payload_size, GFP_KERNEL);
> > > > +	if (!po)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	pi.offset = cpu_to_le64(offset);
> > > > +	pi.length = cpu_to_le64(len);
> > > > +
> > > > +	rc = mutex_lock_interruptible(&cxlds->poison_list_mutex);
> > > 
> > > So I do not know what this mutex is protecting if there is an allocation
> > > per cxl_mem_get_poison() invocation. Although I suspect that's somewhat
> > > wasteful. Just allocate one buffer at the beginning of time and then use
> > > the lock to protect that buffer.
> > 
> > Intent was a single lock on the device to protect the state of the
> > poison list retrieval - do not allow > 1 reader. With > 1 reader
> > software may not know if it retrieved the complete list.
> > 
> > I'm not understanding about protecting a buffer, instead of protecting
> > the state. Here I try to protect the state.
> 
> Ah, sorry I read cxlds->poison_list_mutex and assumed it was serializing
> access to the buffer, not a state machine. I do think it would be
> worthwhile to make this a self contained structure with its own kdoc to
> explain what is going on, something like:
> 

OK thanks, it's getting through to me now. 
Will do !

> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
> index ab138004f644..02697d1d951c 100644
> --- a/drivers/cxl/cxlmem.h
> +++ b/drivers/cxl/cxlmem.h
> @@ -193,6 +193,19 @@ struct cxl_endpoint_dvsec_info {
>         struct range dvsec_range[2];
>  };
>  
> +/**
> + * struct cxl_poison_state - summary
> + * @payload: ...
> + * @lock: ...
> + *
> + * A bit more description of why state needs to be held over successive
> + * mbox commands...
> + */
> +struct cxl_poison_state {
> +       void *payload;
> +       struct mutex lock;
> +};
> +
>  /**
>   * struct cxl_dev_state - The driver device state
>   *
> @@ -210,6 +223,7 @@ struct cxl_endpoint_dvsec_info {
>   * @lsa_size: Size of Label Storage Area
>   *                (CXL 2.0 8.2.9.5.1.1 Identify Memory Device)
>   * @mbox_mutex: Mutex to synchronize mailbox access.
> + * @poison: Poison list retrieval and tracing
>   * @firmware_version: Firmware version for the memory device.
>   * @enabled_cmds: Hardware commands found enabled in CEL.
>   * @exclusive_cmds: Commands that are kernel-internal only
> @@ -244,6 +258,7 @@ struct cxl_dev_state {
>         size_t payload_size;
>         size_t lsa_size;
>         struct mutex mbox_mutex; /* Protects device mailbox and firmware */
> +       struct cxl_poison_state poison;
>         char firmware_version[0x10];
>         DECLARE_BITMAP(enabled_cmds, CXL_MEM_COMMAND_ID_MAX);
>         DECLARE_BITMAP(exclusive_cmds, CXL_MEM_COMMAND_ID_MAX);