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

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Date:   Wed, 7 Dec 2022 13:39:58 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Alison Schofield <alison.schofield@...el.com>,
        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

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:

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);