On Tue, Oct 29, 2019 at 11:28 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > > > On Tue, 29 Oct 2019, Shakeel Butt wrote: > > > +Marco > > > > On Tue, Oct 29, 2019 at 2:03 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > > > On Mon 28-10-19 17:54:05, Shakeel Butt wrote: > > > > Syzbot reported the following bug: > > > > > > > > BUG: KCSAN: data-race in mem_cgroup_select_victim_node / mem_cgroup_select_victim_node > > > > > > > > write to 0xffff88809fade9b0 of 4 bytes by task 8603 on cpu 0: > > > > mem_cgroup_select_victim_node+0xb5/0x3d0 mm/memcontrol.c:1686 > > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > > > read to 0xffff88809fade9b0 of 4 bytes by task 7290 on cpu 1: > > > > mem_cgroup_select_victim_node+0x92/0x3d0 mm/memcontrol.c:1675 > > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > > > mem_cgroup_select_victim_node() can be called concurrently which reads > > > > and modifies memcg->last_scanned_node without any synchrnonization. So, > > > > read and modify memcg->last_scanned_node with READ_ONCE()/WRITE_ONCE() > > > > to stop potential reordering. > > Strictly speaking, READ_ONCE/WRITE_ONCE alone avoid various bad compiler > optimizations, including store tearing, load tearing, etc. This does not > add memory barriers to constrain memory ordering. (If this code needs > some memory ordering guarantees w.r.t. previous loads/stores then this > alone is not enough.) > > > > I am sorry but I do not understand the problem and the fix. Why does the > > > race happen and why does _ONCE fixes it? There is still no > > > synchronization. Do you want to prevent from memcg->last_scanned_node > > > reloading? > > > > > > > The problem is memcg->last_scanned_node can read and modified > > concurrently. Though to me it seems like a tolerable race and not > > worth to add an explicit lock. My aim was to make KCSAN happy here to > > look elsewhere for the concurrency bugs. However I see that it might > > complain next on memcg->scan_nodes. > > The plain concurrent reads/writes are a data race, which may manifest in > various undefined behaviour due to compiler optimizations. The _ONCE > will prevent these (KCSAN only reports data races). Note that, "data > race" does not necessarily imply "race condition"; some data races are > race conditions (usually the more interesting bugs) -- but not *all* > data races are race conditions. If there is no race condition here that > warrants heavier synchronization (locking etc.), then this patch is all > that should be needed. > > I can't comment on the rest. > Thanks Marco for the explanation.
