The gcc 8.3 compiler generated for the update of the last_tsc
8 byte value two store commands (see __ipipe_update_tsc).
e2500001 subs r0, r0, #1
e5830000 str r0, [r3]
e2c11000 sbc r1, r1, #0
e5831004 str r1, [r3, #4]
This may lead to wrong reading of the counter in the assembler part
__ipipe_tsc_get if a CPU core just reads this value when only half
of the last_tsc field is updated.
__ipipe_freerunning_32:
ldr r0, .LCfr32_cntr_addr
/* User-space entry-point: r0 is the hardware counter virtual address */
myldrd r2, r3, r1, .LCfr32_last_tsc
#ifndef CONFIG_CPU_BIG_ENDIAN
/* Little endian */
ldr r0, [r0]
cmp r2, r0
adc r1, r3, #0
The issue can be seen when enabling DEBUG_TIMEKEEPING option in the
kernel and may lead to a jump backwards in time for the monotonic
timer in Linux.
WARNING: Underflow in clocksource 'ipipe_tsc' observed, time update ignored.
Please report this, consider using a different clocksource, if
possible.
Your kernel is probably still fine.
Fix this by using the WRITE_ONCE macro to prevent the compiler from
tearing the write since this fields are accessed from different CPU
cores. After this change the resulting assembler is using a double store
command, which should fix the issue.
e2506001 subs r6, r0, #1
e2c17000 sbc r7, r1, #0
e1c360f0 strd r6, [r3]
Signed-off-by: Gunter Grau <[email protected]>
---
arch/arm/kernel/ipipe_tsc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/arm/kernel/ipipe_tsc.c b/arch/arm/kernel/ipipe_tsc.c
index 7e58118254b8..6d9e1cefa964 100644
--- a/arch/arm/kernel/ipipe_tsc.c
+++ b/arch/arm/kernel/ipipe_tsc.c
@@ -199,7 +199,7 @@ void __ipipe_tsc_update(void)
/* Update last_tsc, in order to remain compatible with legacy
user-space 32 bits free-running counter implementation */
- ipipe_tsc_value->last_tsc = __ipipe_tsc_get() - 1;
+ WRITE_ONCE(ipipe_tsc_value->last_tsc, __ipipe_tsc_get() - 1);
}
EXPORT_SYMBOL(__ipipe_tsc_get);
--
2.17.1
________________________________
The information contained in this message may be confidential and legally
protected under applicable law. The message is intended solely for the
addressee(s). If you are not the intended recipient, you are hereby notified
that any use, forwarding, dissemination, or reproduction of this message is
strictly prohibited and may be unlawful. If you are not the intended recipient,
please contact the sender by return e-mail and destroy all copies of the
original message.
