From ce9ce6659a5775047ad529ed77ab119da5fb8b0c Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Fri, 4 Sep 2015 15:47:50 -0700 Subject: [PATCH] mm: memcontrol: bring back the VM_BUG_ON() in mem_cgroup_swapout() Clark stumbled over a VM_BUG_ON() in -RT which was then was removed by Johannes in commit f371763a79d ("mm: memcontrol: fix false-positive VM_BUG_ON() on -rt"). The comment before that patch was a tiny bit better than it is now. While the patch claimed to fix a false-postive on -RT this was not the case. None of the -RT folks ACKed it and it was not a false positive report. That was a *real* problem. This patch updates the comment that is improper because it refers to "disabled preemption" as a consequence of that lock being taken. A spin_lock() disables preemption, true, but in this case the code relies on the fact that the lock _also_ disables interrupts once it is acquired. And this is the important detail (which was checked the VM_BUG_ON()) which needs to be pointed out. This is the hint one needs while looking at the code. It was explained by Johannes on the list that the per-CPU variables are protected by local_irq_save(). The BUG_ON() was helpful. This code has been workarounded in -RT in the meantime. I wouldn't mind running into more of those if the code in question uses *special* kind of locking since now there is no verification (in terms of lockdep or BUG_ON()) and therefore I bring the VM_BUG_ON() check back in. The two functions after the comment could also have a "local_irq_save()" dance around them in order to serialize access to the per-CPU variables. This has been avoided because the interrupts should be off. Signed-off-by: Sebastian Andrzej Siewior Acked-by: Johannes Weiner Cc: Thomas Gleixner Cc: Steven Rostedt Cc: Clark Williams Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index acb93c554f6e..1af057575ce9 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5965,7 +5965,13 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry) if (!mem_cgroup_is_root(memcg)) page_counter_uncharge(&memcg->memory, 1); - /* Caller disabled preemption with mapping->tree_lock */ + /* + * Interrupts should be disabled here because the caller holds the + * mapping->tree_lock lock which is taken with interrupts-off. It is + * important here to have the interrupts disabled because it is the + * only synchronisation we have for udpating the per-CPU variables. + */ + VM_BUG_ON(!irqs_disabled()); mem_cgroup_charge_statistics(memcg, page, -1); memcg_check_events(memcg, page); } -- 2.11.0