[PATCH] x86, microcode: Add local mutex to not hit a deadlock.

Konrad Rzeszutek Wilk
2013-05-08T16:13:50Z

This can easily be triggered if a new CPU is added (via
ACPI hotplug mechanism) and from user-space do:

echo 1 > /sys/devices/system/cpu/cpu3/online

(or wait for UDEV to do it) on a newly appeared CPU.

The deadlock is that the "store_online" in drivers/base/cpu.c
takes the cpu_hotplug_driver_lock() lock, then calls "cpu_up".
"cpu_up" eventually ends up calling "save_mc_for_early"
which also takes the cpu_hotplug_driver_lock() lock.

And here is that kernel thinks of it:

smpboot: Stack at about ffff880075c39f44
smpboot: CPU3: has booted.
microcode: CPU3 sig=0x206a7, pf=0x2, revision=0x25

=============================================
[ INFO: possible recursive locking detected ]
3.9.0upstream-10129-g167af0e #1 Not tainted
---------------------------------------------
sh/2487 is trying to acquire lock:
(x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20

but task is already holding lock:
(x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(x86_cpu_hotplug_driver_mutex);
lock(x86_cpu_hotplug_driver_mutex);

*** DEADLOCK ***

May be due to missing lock nesting notation

6 locks held by sh/2487:
#0: (sb_writers#5){.+.+.+}, at: [<ffffffff811ca48d>] vfs_write+0x17d/0x190
#1: (&buffer->mutex){+.+.+.}, at: [<ffffffff812464ef>] sysfs_write_file+0x3f/0x160
#2: (s_active#20){.+.+.+}, at: [<ffffffff81246578>] sysfs_write_file+0xc8/0x160
#3: (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] cpu_hotplug_driver_lock+0x12/0x20
#4: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff810961c2>] cpu_maps_update_begin+0x12/0x20
#5: (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff810962a7>] cpu_hotplug_begin+0x27/0x60

Suggested-by: Borislav Petkov <bp@alien8.de>
CC: stable@vger.kernel.org # for v3.9
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
arch/x86/kernel/microcode_intel_early.c | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/microcode_intel_early.c b/arch/x86/kernel/microcode_intel_early.c
index d893e8e..2e9e128 100644
--- a/arch/x86/kernel/microcode_intel_early.c
+++ b/arch/x86/kernel/microcode_intel_early.c
@@ -487,6 +487,7 @@ static inline void show_saved_mc(void)
#endif

#if defined(CONFIG_MICROCODE_INTEL_EARLY) && defined(CONFIG_HOTPLUG_CPU)
+static DEFINE_MUTEX(x86_cpu_microcode_mutex);
/*
* Save this mc into mc_saved_data. So it will be loaded early when a CPU is
* hot added or resumes.
@@ -507,7 +508,7 @@ int save_mc_for_early(u8 *mc)
* Hold hotplug lock so mc_saved_data is not accessed by a CPU in
* hotplug.
*/
- cpu_hotplug_driver_lock();
+ mutex_lock(&x86_cpu_microcode_mutex);

mc_saved_count_init = mc_saved_data.mc_saved_count;
mc_saved_count = mc_saved_data.mc_saved_count;
@@ -544,7 +545,7 @@ int save_mc_for_early(u8 *mc)
}

out:
- cpu_hotplug_driver_unlock();
+ mutex_unlock(&x86_cpu_microcode_mutex);

return ret;
}
--
1.7.7.6