powerpc/smp: Call smp_ops->setup_cpu() directly on the boot CPU

Michael Ellerman patch-notifications at ellerman.id.au
Mon Jul 31 16:31:41 AEST 2017

On Thu, 2017-07-27 at 13:23:37 UTC, Michael Ellerman wrote:
> In smp_cpus_done() we need to call smp_ops->setup_cpu() for the boot
> CPU, which means it has to run *on* the boot CPU.
> In the past we ensured it ran on the boot CPU by changing the CPU
> affinity mask of current directly. That was removed in commit
> 6d11b87d55eb ("powerpc/smp: Replace open coded task affinity logic"),
> and replaced with a work queue call.
> Unfortunately using a work queue leads to a lockdep warning, now that
> the CPU hotplug lock is a regular semaphore:
>   ======================================================
>   WARNING: possible circular locking dependency detected
>   ...
>   kworker/0:1/971 is trying to acquire lock:
>    (cpu_hotplug_lock.rw_sem){++++++}, at: [<c000000000100974>] apply_workqueue_attrs+0x34/0xa0
>   but task is already holding lock:
>    ((&wfc.work)){+.+.+.}, at: [<c0000000000fdb2c>] process_one_work+0x25c/0x800
>   ...
>        CPU0                    CPU1
>        ----                    ----
>   lock((&wfc.work));
>                                lock(cpu_hotplug_lock.rw_sem);
>                                lock((&wfc.work));
>   lock(cpu_hotplug_lock.rw_sem);
> Although the deadlock can't happen in practice, because
> smp_cpus_done() only runs in early boot before CPU hotplug is allowed,
> lockdep can't tell that.
> Luckily in commit 8fb12156b8db ("init: Pin init task to the boot CPU,
> initially") tglx changed the generic code to pin init to the boot CPU
> to begin with. The unpinning of init from the boot CPU happens in
> sched_init_smp(), which is called after smp_cpus_done().
> So smp_cpus_done() is always called on the boot CPU, which means we
> don't need the work queue call at all - and the lockdep warning goes
> away.
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc fixes.



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