dmaengine: idxd: Fix oops during rmmod on single-CPU platforms

During the removal of the idxd driver, registered offline callback is
invoked as part of the clean up process. However, on systems with only
one CPU online, no valid target is available to migrate the
perf context, resulting in a kernel oops:

    BUG: unable to handle page fault for address: 000000000002a2b8
    #PF: supervisor write access in kernel mode
    #PF: error_code(0x0002) - not-present page
    PGD 1470e1067 P4D 0
    Oops: 0002 [#1] PREEMPT SMP NOPTI
    CPU: 0 PID: 20 Comm: cpuhp/0 Not tainted 6.8.0-rc6-dsa+ #57
    Hardware name: Intel Corporation AvenueCity/AvenueCity, BIOS BHSDCRB1.86B.2492.D03.2307181620 07/18/2023
    RIP: 0010:mutex_lock+0x2e/0x50
    ...
    Call Trace:
    <TASK>
    __die+0x24/0x70
    page_fault_oops+0x82/0x160
    do_user_addr_fault+0x65/0x6b0
    __pfx___rdmsr_safe_on_cpu+0x10/0x10
    exc_page_fault+0x7d/0x170
    asm_exc_page_fault+0x26/0x30
    mutex_lock+0x2e/0x50
    mutex_lock+0x1e/0x50
    perf_pmu_migrate_context+0x87/0x1f0
    perf_event_cpu_offline+0x76/0x90 [idxd]
    cpuhp_invoke_callback+0xa2/0x4f0
    __pfx_perf_event_cpu_offline+0x10/0x10 [idxd]
    cpuhp_thread_fun+0x98/0x150
    smpboot_thread_fn+0x27/0x260
    smpboot_thread_fn+0x1af/0x260
    __pfx_smpboot_thread_fn+0x10/0x10
    kthread+0x103/0x140
    __pfx_kthread+0x10/0x10
    ret_from_fork+0x31/0x50
    __pfx_kthread+0x10/0x10
    ret_from_fork_asm+0x1b/0x30
    <TASK>

Fix the issue by preventing the migration of the perf context to an
invalid target.

Fixes: 81dd4d4 ("dmaengine: idxd: Add IDXD performance monitor support")
Reported-by: Terrence Xu <[email protected]>
Tested-by: Terrence Xu <[email protected]>
Signed-off-by: Fenghua Yu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Vinod Koul <[email protected]>

drivers/dma/idxd/perfmon.c

@@ -528,14 +528,11 @@ static int perf_event_cpu_offline(unsigned int cpu, struct hlist_node *node)
 		return 0;
 
 	target = cpumask_any_but(cpu_online_mask, cpu);
-
 	/* migrate events if there is a valid target */
-	if (target < nr_cpu_ids)
+	if (target < nr_cpu_ids) {
 		cpumask_set_cpu(target, &perfmon_dsa_cpu_mask);
-	else
-		target = -1;
-
-	perf_pmu_migrate_context(&idxd_pmu->pmu, cpu, target);
+		perf_pmu_migrate_context(&idxd_pmu->pmu, cpu, target);
+	}
 
 	return 0;
 }