Re: [PATCH] powerpc/rtas: Fix a potential race between CPU-Offline & Migration
From: Nathan Fontenot <hidden>
Date: 2018-09-27 17:31:42
On 09/27/2018 11:51 AM, Gautham R. Shenoy wrote:
From: "Gautham R. Shenoy" <redacted>
Live Partition Migrations require all the present CPUs to execute the
H_JOIN call, and hence rtas_ibm_suspend_me() onlines any offline CPUs
before initiating the migration for this purpose.
The commit 85a88cabad57
("powerpc/pseries: Disable CPU hotplug across migrations")
disables any CPU-hotplug operations once all the offline CPUs are
brought online to prevent any further state change. Once the
CPU-Hotplug operation is disabled, the code assumes that all the CPUs
are online.
However, there is a minor window in rtas_ibm_suspend_me() between
onlining the offline CPUs and disabling CPU-Hotplug when a concurrent
CPU-offline operations initiated by the userspace can succeed thereby
nullifying the the aformentioned assumption. In this unlikely case
these offlined CPUs will not call H_JOIN, resulting in a system hang.
Fix this by verifying that all the present CPUs are actually online
after CPU-Hotplug has been disabled, failing which we return from
rtas_ibm_suspend_me() with -EBUSY.Would we also want to havr the ability to re-try onlining all of the cpus before failing the migration? This would involve a bigger code change as the current code to online all CPUs would work in its current form. -Nathan
quoted hunk ↗ jump to hunk
Cc: Nathan Fontenot <redacted> Cc: Tyrel Datwyler <redacted> Suggested-by: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Gautham R. Shenoy <redacted> --- arch/powerpc/kernel/rtas.c | 10 ++++++++++ 1 file changed, 10 insertions(+)diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c index 2c7ed31..27f6fd3 100644 --- a/arch/powerpc/kernel/rtas.c +++ b/arch/powerpc/kernel/rtas.c@@ -982,6 +982,16 @@ int rtas_ibm_suspend_me(u64 handle) } cpu_hotplug_disable(); + + /* Check if we raced with a CPU-Offline Operation */ + if (unlikely(!cpumask_equal(cpu_present_mask, cpu_online_mask))) { + pr_err("%s: Raced against a concurrent CPU-Offline\n", + __func__); + atomic_set(&data.error, -EBUSY); + cpu_hotplug_enable(); + goto out; + } + stop_topology_update(); /* Call function on all CPUs. One of us will make the