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    SubjectRe: [PATCH 1/4] hwtracing: hisi_ptt: Make cpumask only present online CPUs
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    On 2023/3/29 0:24, Jonathan Cameron wrote:
    > On Wed, 15 Mar 2023 17:43:13 +0800
    > Yicong Yang <yangyicong@huawei.com> wrote:
    >
    >> From: Yicong Yang <yangyicong@hisilicon.com>
    >>
    >> perf will try to start PTT trace on every CPU presented in cpumask sysfs
    >> attribute and it will fail to start on offline CPUs(see the comments in
    >> perf_event_open()). But the driver is using cpumask_of_node() to export
    >> the available cpumask which may include offline CPUs and may fail the
    >> perf unintendedly. Fix this by only export the online CPUs of the node.
    >
    > There isn't clear documentation that I can find for cpumask_of_node()
    > and chasing through on arm64 (which is what we care about for this driver)
    > it's maintained via numa_add_cpu() numa_remove_cpu()
    > Those are called in arch/arm64/kernel/smp.c in locations that are closely coupled
    > with set_cpu_online(cpu, XXX);
    > https://elixir.bootlin.com/linux/v6.3-rc4/source/arch/arm64/kernel/smp.c#L246
    > https://elixir.bootlin.com/linux/v6.3-rc4/source/arch/arm64/kernel/smp.c#L303
    >
    > Now there are races when the two might not be in sync but in this case
    > we are just exposing the result to userspace, so chances of a race
    > after this sysfs attribute has been read seems much higher to me and
    > I don't think we can do anything about that.
    >
    > Is there another path that I'm missing where online and node masks are out
    > of sync?
    >

    maybe no. This patch maybe incorrect and I need more investigation, so let's me
    drop it from the series. Tested and everything seems fine now.

    I found this problem and referred to commit 064f0e9302af ("mm: only display online cpus of the numa node")
    which might be the same problem. But seems unnecessary that cpumask_of_node()
    already include online CPUs only.

    Thanks.

    > Jonathan
    >
    >
    >>
    >> Fixes: ff0de066b463 ("hwtracing: hisi_ptt: Add trace function support for HiSilicon PCIe Tune and Trace device")
    >> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
    >
    >> ---
    >> drivers/hwtracing/ptt/hisi_ptt.c | 13 +++++++++++--
    >> 1 file changed, 11 insertions(+), 2 deletions(-)
    >>
    >> diff --git a/drivers/hwtracing/ptt/hisi_ptt.c b/drivers/hwtracing/ptt/hisi_ptt.c
    >> index 30f1525639b5..0a10c7ec46ad 100644
    >> --- a/drivers/hwtracing/ptt/hisi_ptt.c
    >> +++ b/drivers/hwtracing/ptt/hisi_ptt.c
    >> @@ -487,9 +487,18 @@ static ssize_t cpumask_show(struct device *dev, struct device_attribute *attr,
    >> char *buf)
    >> {
    >> struct hisi_ptt *hisi_ptt = to_hisi_ptt(dev_get_drvdata(dev));
    >> - const cpumask_t *cpumask = cpumask_of_node(dev_to_node(&hisi_ptt->pdev->dev));
    >> + cpumask_var_t mask;
    >> + ssize_t n;
    >>
    >> - return cpumap_print_to_pagebuf(true, buf, cpumask);
    >> + if (!alloc_cpumask_var(&mask, GFP_KERNEL))
    >> + return 0;
    >> +
    >> + cpumask_and(mask, cpumask_of_node(dev_to_node(&hisi_ptt->pdev->dev)),
    >> + cpu_online_mask);
    >> + n = cpumap_print_to_pagebuf(true, buf, mask);
    >> + free_cpumask_var(mask);
    >> +
    >> + return n;
    >> }
    >> static DEVICE_ATTR_RO(cpumask);
    >>
    >
    > .
    >

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