On Fri, 21 Feb 2020, Qian Cai wrote:
> "vm_committed_as.count" could be accessed concurrently as reported by
> KCSAN,
>
> BUG: KCSAN: data-race in __vm_enough_memory / percpu_counter_add_batch
>
> write to 0xffffffff9451c538 of 8 bytes by task 65879 on cpu 35:
> percpu_counter_add_batch+0x83/0xd0
> percpu_counter_add_batch at lib/percpu_counter.c:91
> __vm_enough_memory+0xb9/0x260
> dup_mm+0x3a4/0x8f0
> copy_process+0x2458/0x3240
> _do_fork+0xaa/0x9f0
> __do_sys_clone+0x125/0x160
> __x64_sys_clone+0x70/0x90
> do_syscall_64+0x91/0xb05
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> read to 0xffffffff9451c538 of 8 bytes by task 66773 on cpu 19:
> __vm_enough_memory+0x199/0x260
> percpu_counter_read_positive at include/linux/percpu_counter.h:81
> (inlined by) __vm_enough_memory at mm/util.c:839
> mmap_region+0x1b2/0xa10
> do_mmap+0x45c/0x700
> vm_mmap_pgoff+0xc0/0x130
> ksys_mmap_pgoff+0x6e/0x300
> __x64_sys_mmap+0x33/0x40
> do_syscall_64+0x91/0xb05
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> The read is outside percpu_counter::lock critical section which results
> in a data race. Fix it by adding a READ_ONCE() in
> percpu_counter_read_positive() which could also service as the existing
> compiler memory barrier.
>
> Signed-off-by: Qian Cai <cai@xxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
FWIW in the function where this was inlined here, the generated code (on
x86 at least) is identical.
Thanks,
-- Marco
> ---
> include/linux/percpu_counter.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 4f052496cdfd..0a4f54dd4737 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -78,9 +78,9 @@ static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> */
> static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
> {
> - s64 ret = fbc->count;
> + /* Prevent reloads of fbc->count */
> + s64 ret = READ_ONCE(fbc->count);
>
> - barrier(); /* Prevent reloads of fbc->count */
> if (ret >= 0)
> return ret;
> return 0;
> --
> 1.8.3.1
>
