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Subject[PATCH] mm/swap: annotate data races for lru_rotate_pvecs
Date
Read to lru_add_pvec->nr could be interrupted and then write to the same
variable. The write has local interrupt disabled, but the plain reads
result in data races. However, it is unlikely the compilers could
do much damage here given that lru_add_pvec->nr is a "unsigned char" and
there is an existing compiler barrier. Thus, annotate the reads using the
data_race() macro. The data races were reported by KCSAN,

BUG: KCSAN: data-race in lru_add_drain_cpu / rotate_reclaimable_page

write to 0xffff9291ebcb8a40 of 1 bytes by interrupt on cpu 23:
rotate_reclaimable_page+0x2df/0x490
pagevec_add at include/linux/pagevec.h:81
(inlined by) rotate_reclaimable_page at mm/swap.c:259
end_page_writeback+0x1b5/0x2b0
end_swap_bio_write+0x1d0/0x280
bio_endio+0x297/0x560
dec_pending+0x218/0x430 [dm_mod]
clone_endio+0xe4/0x2c0 [dm_mod]
bio_endio+0x297/0x560
blk_update_request+0x201/0x920
scsi_end_request+0x6b/0x4a0
scsi_io_completion+0xb7/0x7e0
scsi_finish_command+0x1ed/0x2a0
scsi_softirq_done+0x1c9/0x1d0
blk_done_softirq+0x181/0x1d0
__do_softirq+0xd9/0x57c
irq_exit+0xa2/0xc0
do_IRQ+0x8b/0x190
ret_from_intr+0x0/0x42
delay_tsc+0x46/0x80
__const_udelay+0x3c/0x40
__udelay+0x10/0x20
kcsan_setup_watchpoint+0x202/0x3a0
__tsan_read1+0xc2/0x100
lru_add_drain_cpu+0xb8/0x3f0
lru_add_drain+0x25/0x40
shrink_active_list+0xe1/0xc80
shrink_lruvec+0x766/0xb70
shrink_node+0x2d6/0xca0
do_try_to_free_pages+0x1f7/0x9a0
try_to_free_pages+0x252/0x5b0
__alloc_pages_slowpath+0x458/0x1290
__alloc_pages_nodemask+0x3bb/0x450
alloc_pages_vma+0x8a/0x2c0
do_anonymous_page+0x16e/0x6f0
__handle_mm_fault+0xcd5/0xd40
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40

read to 0xffff9291ebcb8a40 of 1 bytes by task 37761 on cpu 23:
lru_add_drain_cpu+0xb8/0x3f0
lru_add_drain_cpu at mm/swap.c:602
lru_add_drain+0x25/0x40
shrink_active_list+0xe1/0xc80
shrink_lruvec+0x766/0xb70
shrink_node+0x2d6/0xca0
do_try_to_free_pages+0x1f7/0x9a0
try_to_free_pages+0x252/0x5b0
__alloc_pages_slowpath+0x458/0x1290
__alloc_pages_nodemask+0x3bb/0x450
alloc_pages_vma+0x8a/0x2c0
do_anonymous_page+0x16e/0x6f0
__handle_mm_fault+0xcd5/0xd40
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40

2 locks held by oom02/37761:
#0: ffff9281e5928808 (&mm->mmap_sem#2){++++}, at: do_page_fault
#1: ffffffffb3ade380 (fs_reclaim){+.+.}, at: fs_reclaim_acquire.part
irq event stamp: 1949217
trace_hardirqs_on_thunk+0x1a/0x1c
__do_softirq+0x2e7/0x57c
__do_softirq+0x34c/0x57c
irq_exit+0xa2/0xc0

Reported by Kernel Concurrency Sanitizer on:
CPU: 23 PID: 37761 Comm: oom02 Not tainted 5.6.0-rc3-next-20200226+ #6
Hardware name: HP ProLiant BL660c Gen9, BIOS I38 10/17/2018

Signed-off-by: Qian Cai <cai@lca.pw>
---

BTW, while at it, I had also looked at other pagevec there, but could
not tell for sure if they could be interrupted resulting in data races,
so I leave them out for now.

mm/swap.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index cf39d24ada2a..c922f99dab85 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -599,7 +599,8 @@ void lru_add_drain_cpu(int cpu)
__pagevec_lru_add(pvec);

pvec = &per_cpu(lru_rotate_pvecs, cpu);
- if (pagevec_count(pvec)) {
+ /* Disabling interrupts below acts as a compiler barrier. */
+ if (data_race(pagevec_count(pvec))) {
unsigned long flags;

/* No harm done if a racing interrupt already did this */
@@ -744,7 +745,7 @@ void lru_add_drain_all(void)
struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);

if (pagevec_count(&per_cpu(lru_add_pvec, cpu)) ||
- pagevec_count(&per_cpu(lru_rotate_pvecs, cpu)) ||
+ data_race(pagevec_count(&per_cpu(lru_rotate_pvecs, cpu))) ||
pagevec_count(&per_cpu(lru_deactivate_file_pvecs, cpu)) ||
pagevec_count(&per_cpu(lru_deactivate_pvecs, cpu)) ||
pagevec_count(&per_cpu(lru_lazyfree_pvecs, cpu)) ||
--
2.21.0 (Apple Git-122.2)
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