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Subject[PATCH] asm-generic/futex: Properly re-enable preemption in futex_atomic_cmpxchg_inatomic()
Date
The futex subsystem relies on futex_atomic_cmpxchg_inatomic() to
perform an atomic cmpxchg. Most architectures provide their own
implementation. For those who do not, <asm-generic/futex.h> provides a
generic software implementations that works on !SMP platforms.

To provide an atomicity guarantee on !SMP plaforms, the generic
implementation simply disables preemption.
However it is not properly re-enabled from conditional branches.

The observed consequence of this bug is that the preemption counter is
not well balanced. It can be seen at boot time :

[ 8.148000] futex hash table entries: 256 (order: -1, 3072 bytes)
[ 8.156000] ------------[ cut here ]------------
[ 8.164000] WARNING: CPU: 0 PID: 1 at init/main.c:806 do_one_initcall+0x1ac/0x1d0()
[ 8.172000] initcall futex_init+0x0/0xd8 returned with preemption imbalance
[ 8.180000] Modules linked in:
[ 8.184000] CPU: 0 PID: 1 Comm: swapper Not tainted 4.5.0 #72
[ 8.192000] Stack from d1c1deb8:
[ 8.196000]
[ 8.196000] d1c1debc
[ 8.204000] d1c1defc 00000009
[ 8.204000] d04d8b90 00000326
[ 8.208000] d0000380 d0136484
[ 8.212000] d00093b4

[ 8.216000] 00000326 d0000380
[ 8.220000] d1c1c000 00000000
[ 8.224000] d026c64c 00000000
[ 8.228000] d0515004 d0009410
[ 8.232000]
[ 8.232000] d1c1defc
[ 8.236000] d04d8d8c d1c1df08
[ 8.240000] d0000380 d026c64c
[ 8.244000] d1c1df10 65657270
[ 8.248000] 6974706d

[ 8.252000] 69206e6f 6c61626d
[ 8.256000] 65636e61 d0780020
[ 8.260000] d05108a0 00000001
[ 8.264000] d00225c8 00000000
[ 8.268000]
[ 8.268000] d05103c8
[ 8.272000] 0000003f 00060006
[ 8.276000] d05108a0 00000006
[ 8.280000] 0000003f 00000007
[ 8.284000] 0000003f

[ 8.288000] d052501c d0525024
[ 8.288000] d027d480 d0266c78
[ 8.292000] 00000006 00000006
[ 8.296000] 00000000 d026627c
[ 8.300000]
[ 8.300000] Call Trace:

[ 8.308000] [<d025fc58>] [<d025fc68>]
[ 8.312000] [<d025fc58>] [<d000130c>]
[ 8.316000]
[ 8.316000]

---[ end trace 2d3fe47ac46f91f6 ]---

Before commit 8222dbe21e79
("sched/preempt, mm/fault: Decouple preemption from the page fault logic")
, the preemption was disabled/re-enabled by the
pagefault_disable()/pagefault_enable() functions. It is no longer the
case.

Which means that cmpxchg_futex_value_locked(), in kernel/futex.c, no
longer disables preemption before calling futex_atomic_cmpxchg_inatomic.
Disabling preemption in the generic implementation was added by commit
d9b9ff8c1889 ("sched/preempt, futex: Disable preemption in UP futex_atomic_cmpxchg_inatomic() explicitly").
However, this change is bogus: preemption is not re-enabled in the two failure cases.

Indeed, At boot time futex_detect_cmpxchg(), in kernel/futex.c, checks if
futex_atomic_cmpxchg_inatomic() operation is provided by the current
architecture. To do so, it calls futex_atomic_cmpxchg_inatomic() with a
NULL pointer as the futex, and expects -EFAULT as a return value. The
reasoning if that if -EFAULT is returned, then there is a valid
futex_atomic_cmpxchg_inatomic() implementation, while if -ENOSYS is
returned, then it means that there is no implementation of this function
for the current architecture.

By doing this test based on a NULL pointer, we fall into the case where
the get_user() in the asm-generic version of
futex_atomic_cmpxchg_inatomic() fails, and returns -EFAULT without
re-enabling the preemption, causing the preemption unbalance in the
futex_init() initcall.

This bug was not found, because most of the architectures in the kernel
use their own implementation. This is not the case for the nios2 platform.
By using a nios2 board without SMP support, this bug can be easily
reproduced.

Fixes: d9b9ff8c1889 ("sched/preempt, futex: Disable preemption in UP futex_atomic_cmpxchg_inatomic() explicitly")
Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
---
include/asm-generic/futex.h | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/futex.h b/include/asm-generic/futex.h
index e56272c..bf2d34c 100644
--- a/include/asm-generic/futex.h
+++ b/include/asm-generic/futex.h
@@ -108,11 +108,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
u32 val;

preempt_disable();
- if (unlikely(get_user(val, uaddr) != 0))
+ if (unlikely(get_user(val, uaddr) != 0)) {
+ preempt_enable();
return -EFAULT;
+ }

- if (val == oldval && unlikely(put_user(newval, uaddr) != 0))
+ if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) {
+ preempt_enable();
return -EFAULT;
+ }

*uval = val;
preempt_enable();
--
2.8.0
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