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    Subject[PATCH] x86/resctrl: Ensure usage of CPUs are locked while needed
    Date
    The user triggers the creation of a pseudo-locked region when writing
    the requested schemata to the schemata resctrl file. The pseudo-locking
    of a region is required to be done on a CPU that is associated with the
    cache on which the pseudo-locked region will reside. In order to run the
    locking code on a specific CPU the needed CPU has to be selected and
    ensured to remain online during the entire locking sequence.

    At this time the cpu_hotplug_lock is not taken during the pseudo-lock
    region creation and it is thus possible for a CPU to be selected to run
    the pseudo-locking code and then that CPU to go offline before the
    thread is able to run on it.

    Fix this by ensuring that the cpu_hotplug_lock is taken while the CPU on
    which code has to run needs to be controlled. Since the cpu_hotplug_lock
    is always taken before rdtgroup_mutex the lock order is maintained.

    Fixes: e0bdfe8e36f3 ("x86/intel_rdt: Support creation/removal of pseudo-locked region")
    Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
    Cc: stable@vger.kernel.org
    ---
    arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
    index 03ee13235a45..6f7adb3be01e 100644
    --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
    +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
    @@ -23,6 +23,7 @@

    #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

    +#include <linux/cpu.h>
    #include <linux/kernfs.h>
    #include <linux/seq_file.h>
    #include <linux/slab.h>
    @@ -381,9 +382,11 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
    return -EINVAL;
    buf[nbytes - 1] = '\0';

    + cpus_read_lock();
    rdtgrp = rdtgroup_kn_lock_live(of->kn);
    if (!rdtgrp) {
    rdtgroup_kn_unlock(of->kn);
    + cpus_read_unlock();
    return -ENOENT;
    }
    rdt_last_cmd_clear();
    @@ -438,6 +441,7 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,

    out:
    rdtgroup_kn_unlock(of->kn);
    + cpus_read_unlock();
    return ret ?: nbytes;
    }

    --
    2.17.0
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