[PATCH v3] cpuset: fix a deadlock due to incomplete patching of cpusets_enabled()

From: Dima Zavin
Date: Mon Jul 31 2017 - 00:01:21 EST

In codepaths that use the begin/retry interface for reading
mems_allowed_seq with irqs disabled, there exists a race condition that
stalls the patch process after only modifying a subset of the
static_branch call sites.

This problem manifested itself as a dead lock in the slub
allocator, inside get_any_partial. The loop reads
mems_allowed_seq value (via read_mems_allowed_begin),
performs the defrag operation, and then verifies the consistency
of mem_allowed via the read_mems_allowed_retry and the cookie
returned by xxx_begin. The issue here is that both begin and retry
first check if cpusets are enabled via cpusets_enabled() static branch.
This branch can be rewritted dynamically (via cpuset_inc) if a new
cpuset is created. The x86 jump label code fully synchronizes across
all CPUs for every entry it rewrites. If it rewrites only one of the
callsites (specifically the one in read_mems_allowed_retry) and then
waits for the smp_call_function(do_sync_core) to complete while a CPU is
inside the begin/retry section with IRQs off and the mems_allowed value
is changed, we can hang. This is because begin() will always return 0
(since it wasn't patched yet) while retry() will test the 0 against
the actual value of the seq counter.

The fix is to use two different static keys: one for begin
(pre_enable_key) and one for retry (enable_key). In cpuset_inc(), we
first bump the pre_enable key to ensure that cpuset_mems_allowed_begin()
always return a valid seqcount if are enabling cpusets. Similarly,
when disabling cpusets via cpuset_dec(), we first ensure that callers
of cpuset_mems_allowed_retry() will start ignoring the seqcount
value before we let cpuset_mems_allowed_begin() return 0.

The relevant stack traces of the two stuck threads:

CPU: 1 PID: 1415 Comm: mkdir Tainted: G L 4.9.36-00104-g540c51286237 #4
Hardware name: Default string Default string/Hardware, BIOS 4.29.1-20170526215256 05/26/2017
task: ffff8817f9c28000 task.stack: ffffc9000ffa4000
RIP: smp_call_function_many+0x1f9/0x260
Call Trace:
? setup_data_read+0xa0/0xa0
? ___slab_alloc+0x28b/0x5a0
smp_call_function+0x3b/0x70
? setup_data_read+0xa0/0xa0
on_each_cpu+0x2f/0x90
? ___slab_alloc+0x28a/0x5a0
? ___slab_alloc+0x28b/0x5a0
text_poke_bp+0x87/0xd0
? ___slab_alloc+0x28a/0x5a0
arch_jump_label_transform+0x93/0x100
__jump_label_update+0x77/0x90
jump_label_update+0xaa/0xc0
static_key_slow_inc+0x9e/0xb0
cpuset_css_online+0x70/0x2e0
online_css+0x2c/0xa0
cgroup_apply_control_enable+0x27f/0x3d0
cgroup_mkdir+0x2b7/0x420
kernfs_iop_mkdir+0x5a/0x80
vfs_mkdir+0xf6/0x1a0
SyS_mkdir+0xb7/0xe0
entry_SYSCALL_64_fastpath+0x18/0xad

...

CPU: 2 PID: 1 Comm: init Tainted: G L 4.9.36-00104-g540c51286237 #4
Hardware name: Default string Default string/Hardware, BIOS 4.29.1-20170526215256 05/26/2017
task: ffff8818087c0000 task.stack: ffffc90000030000
RIP: int3+0x39/0x70
Call Trace:
<#DB> ? ___slab_alloc+0x28b/0x5a0
<EOE> ? copy_process.part.40+0xf7/0x1de0
? __slab_alloc.isra.80+0x54/0x90
? copy_process.part.40+0xf7/0x1de0
? copy_process.part.40+0xf7/0x1de0
? kmem_cache_alloc_node+0x8a/0x280
? copy_process.part.40+0xf7/0x1de0
? _do_fork+0xe7/0x6c0
? _raw_spin_unlock_irq+0x2d/0x60
? trace_hardirqs_on_caller+0x136/0x1d0
? entry_SYSCALL_64_fastpath+0x5/0xad
? do_syscall_64+0x27/0x350
? SyS_clone+0x19/0x20
? do_syscall_64+0x60/0x350
? entry_SYSCALL64_slow_path+0x25/0x25

Reported-by: Cliff Spradlin <cspradlin@xxxxxxxxx>
Signed-off-by: Dima Zavin <dmitriyz@xxxxxxxxx>
---

v3:
- Changed the implementation based on Peter Zijlstra's suggestion. Now
using two keys for begin/retry instead of hacking the state into the
cookie.
- Rebased and tested on top of v4.13-rc3.

v4:
- Moved the cached cpusets_enabled() state into the cookie, turned
the cookie into a struct and updated all the other call sites.
- Applied on top of v4.12 since one of the callers in page_alloc.c changed.
Still only tested on v4.9.36 and compile tested against v4.12.

include/linux/cpuset.h | 19 +++++++++++++++++--
kernel/cgroup/cpuset.c | 1 +
2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 119a3f9604b0..e5a684c04c70 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -18,6 +18,19 @@

#ifdef CONFIG_CPUSETS

+/*
+ * Static branch rewrites can happen in an arbitrary order for a given
+ * key. In code paths where we need to loop with read_mems_allowed_begin() and
+ * read_mems_allowed_retry() to get a consistent view of mems_allowed, we need
+ * to ensure that begin() always gets rewritten before retry() in the
+ * disabled -> enabled transition. If not, then if local irqs are disabled
+ * around the loop, we can deadlock since retry() would always be
+ * comparing the latest value of the mems_allowed seqcount against 0 as
+ * begin() still would see cpusets_enabled() as false. The enabled -> disabled
+ * transition should happen in reverse order for the same reasons (want to stop
+ * looking at real value of mems_allowed.sequence in retry() first).
+ */
+extern struct static_key_false cpusets_pre_enable_key;
extern struct static_key_false cpusets_enabled_key;
static inline bool cpusets_enabled(void)
{
@@ -32,12 +45,14 @@ static inline int nr_cpusets(void)

static inline void cpuset_inc(void)
{
+ static_branch_inc(&cpusets_pre_enable_key);
static_branch_inc(&cpusets_enabled_key);
}

static inline void cpuset_dec(void)
{
static_branch_dec(&cpusets_enabled_key);
+ static_branch_dec(&cpusets_pre_enable_key);
}

extern int cpuset_init(void);
@@ -115,7 +130,7 @@ extern void cpuset_print_current_mems_allowed(void);
*/
static inline unsigned int read_mems_allowed_begin(void)
{
- if (!cpusets_enabled())
+ if (!static_branch_unlikely(&cpusets_pre_enable_key))
return 0;

return read_seqcount_begin(&current->mems_allowed_seq);
@@ -129,7 +144,7 @@ static inline unsigned int read_mems_allowed_begin(void)
*/
static inline bool read_mems_allowed_retry(unsigned int seq)
{
- if (!cpusets_enabled())
+ if (!static_branch_unlikely(&cpusets_enabled_key))
return false;

return read_seqcount_retry(&current->mems_allowed_seq, seq);
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index ca8376e5008c..8d5151688504 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -63,6 +63,7 @@
#include <linux/cgroup.h>
#include <linux/wait.h>

+DEFINE_STATIC_KEY_FALSE(cpusets_pre_enable_key);
DEFINE_STATIC_KEY_FALSE(cpusets_enabled_key);

/* See "Frequency meter" comments, below. */
--
2.14.0.rc0.400.g1c36432dff-goog