[PATCH] s390/time: Use jiffies instead of jiffies_64

[PATCH] s390/time: Use jiffies instead of jiffies_64

[Heiko Carstens]

Christoph Schlameuss and Alexander Egorenkov reported a data-race
reported by KCSAN when jiffies_64 is read:

==================================================================
BUG: KCSAN: data-race in do_account_vtime / tick_do_update_jiffies64

write to 0x0000016599ea8600 of 8 bytes by interrupt on cpu 6:
 tick_do_update_jiffies64+0x140/0x250
=============================================================>
BUG: KCSAN: data-race in do_account_vtime / tick_do_update_ji>

write to 0x0000016599ea8600 of 8 bytes by interrupt on cpu 6:
 tick_do_update_jiffies64+0x140/0x250
 tick_nohz_handler+0x2e6/0x300
 __run_hrtimer+0x156/0x4d0
 __hrtimer_run_queues+0xd2/0x150
...
 system_call+0x72/0x90

read to 0x0000016599ea8600 of 8 bytes by interrupt on cpu 12:
 do_account_vtime+0x7d6/0x860
 vtime_flush+0x26/0xe0
 update_process_times+0x32/0x160
 tick_nohz_handler+0x12a/0x300
...
 system_call+0x72/0x90

value changed: 0x00000000ffffaa6c -> 0x00000000ffffaa6d
...
=============================================================>

Problem is that jiffies_64 instead of jiffies is used. Both are at the
same address, but only jiffies is of volatile type, which prevents this
warning.

Change the vtime code so jiffies instead of jiffies_64 is used
everywhere. This addresses also the inconsistency that both jiffies and
jiffies_64 were used in the original patch which introduced this.

Fixes: f341b8dff982 ("s390/vtime: limit MT scaling value updates")
Reported-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/kernel/vtime.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c
index d804e1140c2e..efcbf406f03e 100644
--- a/arch/s390/kernel/vtime.c
+++ b/arch/s390/kernel/vtime.c
@@ -32,7 +32,7 @@ static atomic64_t virt_timer_elapsed;
 DEFINE_PER_CPU(u64, mt_cycles[8]);
 static DEFINE_PER_CPU(u64, mt_scaling_mult) = { 1 };
 static DEFINE_PER_CPU(u64, mt_scaling_div) = { 1 };
-static DEFINE_PER_CPU(u64, mt_scaling_jiffies);
+static DEFINE_PER_CPU(unsigned long, mt_scaling_jiffies);
 
 static inline void set_vtimer(u64 expires)
 {
@@ -81,7 +81,7 @@ static void update_mt_scaling(void)
 		memcpy(cycles_old, cycles_new,
 		       sizeof(u64) * (smp_cpu_mtid + 1));
 	}
-	__this_cpu_write(mt_scaling_jiffies, jiffies_64);
+	__this_cpu_write(mt_scaling_jiffies, jiffies);
 }
 
 static inline u64 update_tsk_timer(unsigned long *tsk_vtime, u64 new)
@@ -144,7 +144,7 @@ static int do_account_vtime(struct task_struct *tsk)
 		lc->system_timer += timer;
 
 	/* Update MT utilization calculation */
-	if (smp_cpu_mtid && time_after64(jiffies_64, __this_cpu_read(mt_scaling_jiffies)))
+	if (smp_cpu_mtid && time_after(jiffies, __this_cpu_read(mt_scaling_jiffies)))
 		update_mt_scaling();
 
 	/* Calculate cputime delta */
-- 
2.53.0

Re: [PATCH] s390/time: Use jiffies instead of jiffies_64

[sashiko-bot]

> Christoph Schlameuss and Alexander Egorenkov reported a data-race
> reported by KCSAN when jiffies_64 is read:
> 
> ==================================================================
> BUG: KCSAN: data-race in do_account_vtime / tick_do_update_jiffies64
> 
> write to 0x0000016599ea8600 of 8 bytes by interrupt on cpu 6:
>  tick_do_update_jiffies64+0x140/0x250
> =============================================================>
> BUG: KCSAN: data-race in do_account_vtime / tick_do_update_ji>
> 
> write to 0x0000016599ea8600 of 8 bytes by interrupt on cpu 6:
>  tick_do_update_jiffies64+0x140/0x250
>  tick_nohz_handler+0x2e6/0x300
>  __run_hrtimer+0x156/0x4d0
> [ ... ]
> 
> Fixes: f341b8dff982 ("s390/vtime: limit MT scaling value updates")
> Reported-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
> Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260813132505.3691502-1-hca@linux.ibm.com?part=1