Re: [PATCH] asm-generic/futex: Properly re-enable preemption in futex_atomic_cmpxchg_inatomic()

[David Howells]

Romain Perier <romain.perier@xxxxxxxxxxxxxxxxxx> wrote:

> 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@xxxxxxxxxxxxxxxxxx>

Reviewed-by: David Howells <dhowells@xxxxxxxxxx>