On Tue, 2020-02-25 at 20:06 -0800, Matthew Wilcox wrote: > On Tue, Feb 25, 2020 at 10:58:27PM -0500, Qian Cai wrote: > > pgdat->kswapd_classzone_idx could be accessed concurrently in > > wakeup_kswapd(). Plain writes and reads without any lock protection > > result in data races. Fix them by adding a pair of READ|WRITE_ONCE() as > > well as saving a branch (compilers might well optimize the original code > > in an unintentional way anyway). While at it, also take care of > > pgdat->kswapd_order and non-kswapd threads in allow_direct_reclaim(). > > I don't understand why the usages of kswapd_classzone_idx in kswapd() and > kswapd_try_to_sleep() don't need changing too? kswapd_classzone_idx() > looks safe to me, but I'm prone to missing stupid things that compilers > are allowed to do. Right, I did capture the race this time. I'll post a v3. [ 924.803628][ T6299] BUG: KCSAN: data-race in kswapd / wakeup_kswapd [ 924.809949][ T6299] [ 924.812170][ T6299] write to 0xffff90973ffff2dc of 4 bytes by task 820 on cpu 6: [ 924.819630][ T6299] kswapd+0x27c/0x8d0 [ 924.823509][ T6299] kthread+0x1e0/0x200 [ 924.827471][ T6299] ret_from_fork+0x27/0x50 [ 924.831774][ T6299] [ 924.833987][ T6299] read to 0xffff90973ffff2dc of 4 bytes by task 6299 on cpu 0: [ 924.841442][ T6299] wakeup_kswapd+0xf3/0x450 [ 924.845838][ T6299] wake_all_kswapds+0x59/0xc0 [ 924.850409][ T6299] __alloc_pages_slowpath+0xdcc/0x1290 [ 924.855769][ T6299] __alloc_pages_nodemask+0x3bb/0x450 [ 924.861040][ T6299] alloc_pages_vma+0x8a/0x2c0 [ 924.865612][ T6299] do_anonymous_page+0x170/0x700 [ 924.870443][ T6299] __handle_mm_fault+0xc9f/0xd00 [ 924.875276][ T6299] handle_mm_fault+0xfc/0x2f0 [ 924.879849][ T6299] do_page_fault+0x263/0x6f9 [ 924.884334][ T6299] page_fault+0x34/0x40
