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    SubjectRe: [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);
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