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s390: fix preemption race in disable_sacf_uaccess 

With CONFIG_PREEMPT=y there is a possible race in disable_sacf_uaccess.

The new set_fs value needs to be stored the the task structure first,
the control register update needs to be second. Otherwise a preemptive
schedule may interrupt the code right after the control register update
has been done and the next time the task is scheduled we get an incorrect
value in the control register due to the old set_fs setting.

Fixes: 0aaba41b58 ("s390: remove all code using the access register mode")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
This commit is contained in:
Martin Schwidefsky 2017-12-15 14:16:04 +01:00
parent 366d821648
commit 9f37e79754
1 changed files with 1 additions and 1 deletions
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2
arch/s390/lib/uaccess.c
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@ -89,11 +89,11 @@ EXPORT_SYMBOL(enable_sacf_uaccess);
void disable_sacf_uaccess(mm_segment_t old_fs)
{
current->thread.mm_segment = old_fs;
if (old_fs == USER_DS && test_facility(27)) {
__ctl_load(S390_lowcore.user_asce, 1, 1);
clear_cpu_flag(CIF_ASCE_PRIMARY);
}
current->thread.mm_segment = old_fs;
}
EXPORT_SYMBOL(disable_sacf_uaccess);