x86/intel_rdt: Prevent deadlock against hotplug lock

The cpu online/offline callbacks of intel_rdt lock rdtgroup_mutex nested
inside of cpu hotplug lock. rdtgroup_cpus_write() does it in reverse order.

Remove the get/put_online_cpus() calls from rdtgroup_cpus_write(). This is
safe against cpu hotplug as the resource group cpumasks are protected by
rdtgroup_mutex.

Found by review, but should have been found if authors would have bothered
to test cpu hotplug with lockdep enabled.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Shaohua Li <shli@fb.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Tony Luck <tony.luck@intel.com>

diff --git a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
index cff286e6d69f55aecebe94102b92906f6ad40f20..2f54931e0fa9ff4365cf313d70d5a86bf016dcc2 100644 (file)
--- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
+++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
@@ -207,6 +207,7 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
                free_cpumask_var(tmpmask);
                return -ENOMEM;
        }
+
        rdtgrp = rdtgroup_kn_lock_live(of->kn);
        if (!rdtgrp) {
                ret = -ENOENT;
@@ -217,12 +218,11 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
        if (ret)
                goto unlock;
 
-       get_online_cpus();
        /* check that user didn't specify any offline cpus */
        cpumask_andnot(tmpmask, newmask, cpu_online_mask);
        if (cpumask_weight(tmpmask)) {
                ret = -EINVAL;
-               goto end;
+               goto unlock;
        }
 
        /* Check whether cpus are dropped from this group */
@@ -231,7 +231,7 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
                /* Can't drop from default group */
                if (rdtgrp == &rdtgroup_default) {
                        ret = -EINVAL;
-                       goto end;
+                       goto unlock;
                }
                /* Give any dropped cpus to rdtgroup_default */
                cpumask_or(&rdtgroup_default.cpu_mask,
@@ -258,8 +258,6 @@ static ssize_t rdtgroup_cpus_write(struct kernfs_open_file *of,
        /* Done pushing/pulling - update this group with new mask */
        cpumask_copy(&rdtgrp->cpu_mask, newmask);
 
-end:
-       put_online_cpus();
 unlock:
        rdtgroup_kn_unlock(of->kn);
        free_cpumask_var(tmpmask);