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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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@xxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx # for v3.9
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 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


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