[PATCH v2] ARM: Fix preemption disable in iwmmxt_task_enable()

From: Sebastian Hesselbarth
Date: Sat Jul 12 2014 - 07:05:55 EST

commit 431a84b1a4f7d1a0085d5b91330c5053cc8e8b12
("ARM: 8034/1: Disable preemption in iwmmxt_task_enable()")
introduced macros {inc,dec}_preempt_count to iwmmxt_task_enable
to make it run with preemption disabled.

Unfortunately, other functions in iwmmxt.S also use concan_{save,dump,load}
sections located in iwmmxt_task_enable() to deal with iWMMXt coprocessor.
This causes an unbalanced preempt_count due to excessive dec_preempt_count
and destroyed return addresses in callers of concan_ labels due to a register

Linux version 3.16.0-rc3-00062-gd92a333-dirty (jef@armhf) (gcc version 4.8.3 (Debian 4.8.3-4) ) #5 PREEMPT Thu Jul 3 19:46:39 CEST 2014
CPU: ARMv7 Processor [560f5815] revision 5 (ARMv7), cr=10c5387d
CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
Machine model: SolidRun CuBox
PJ4 iWMMXt v2 coprocessor enabled.
Unable to handle kernel paging request at virtual address fffffffe
pgd = bb25c000
[fffffffe] *pgd=3bfde821, *pte=00000000, *ppte=00000000
Internal error: Oops: 80000007 [#1] PREEMPT ARM
Modules linked in:
CPU: 0 PID: 62 Comm: startpar Not tainted 3.16.0-rc3-00062-gd92a333-dirty #5
task: bb230b80 ti: bb256000 task.ti: bb256000
PC is at 0xfffffffe
LR is at iwmmxt_task_copy+0x44/0x4c
pc : [<fffffffe>] lr : [<800130ac>] psr: 40000033
sp : bb257de8 ip : 00000013 fp : bb257ea4
r10: bb256000 r9 : fffffdfe r8 : 76e898e6
r7 : bb257ec8 r6 : bb256000 r5 : 7ea12760 r4 : 000000a0
r3 : ffffffff r2 : 00000003 r1 : bb257df8 r0 : 00000000
Flags: nZcv IRQs on FIQs on Mode SVC_32 ISA Thumb Segment user
Control: 10c5387d Table: 3b25c019 DAC: 00000015
Process startpar (pid: 62, stack limit = 0xbb256248)

This patch fixes the issue by moving concan_{save,dump,load} into separate
code sections and make iwmmxt_task_enable() call them in the same way the
other functions use concan_ symbols. The test for valid ownership is moved
to concan_save and is safe for the other user of it, iwmmxt_task_disable().
The register collision is also resolved by moving concan_ symbols as
{inc,dec}_preempt_count are now local to iwmmxt_task_enable().

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@xxxxxxxxx>
Reported-by: Jean-Francois Moine <moinejf@xxxxxxx>
Fixes: 431a84b1a4f7 ("ARM: 8034/1: Disable preemption in iwmmxt_task_enable()")
The offending commit was intoduced past v3.15-rc1 and the corresponding fix
should also be queued up for stable v3.15+

- return immediately from concan_ instead of branch to 3f, i.e. replace
'beq 3f' with 'moveq pc, lr' (Suggested by Catalin Marinas)

Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Jean-Francois Moine <moinejf@xxxxxxx>
Cc: Jason Cooper <jason@xxxxxxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
arch/arm/kernel/iwmmxt.S | 23 ++++++++++++-----------
1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/arch/arm/kernel/iwmmxt.S b/arch/arm/kernel/iwmmxt.S
index a5599cfc43cb..2b32978ae905 100644
--- a/arch/arm/kernel/iwmmxt.S
+++ b/arch/arm/kernel/iwmmxt.S
@@ -94,13 +94,19 @@ ENTRY(iwmmxt_task_enable)

mrc p15, 0, r2, c2, c0, 0
mov r2, r2 @ cpwait
+ bl concan_save

- teq r1, #0 @ test for last ownership
- mov lr, r9 @ normal exit from exception
- beq concan_load @ no owner, skip save
+ get_thread_info r10
+4: dec_preempt_count r10, r3
+ mov pc, r9 @ normal exit from exception


+ teq r1, #0 @ test for last ownership
+ beq concan_load @ no owner, skip save
tmrc r2, wCon

@ CUP? wCx
@@ -138,7 +144,7 @@ concan_dump:
wstrd wR15, [r1, #MMX_WR15]

2: teq r0, #0 @ anything to load?
- beq 3f
+ moveq pc, lr @ if not, return


@@ -171,14 +177,9 @@ concan_load:
@ clear CUP/MUP (only if r1 != 0)
teq r1, #0
mov r2, #0
- beq 3f
- tmcr wCon, r2
+ moveq pc, lr

- get_thread_info r10
-4: dec_preempt_count r10, r3
+ tmcr wCon, r2
mov pc, lr


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/