On Tue 03-08-21 10:15:36, Johannes Weiner wrote: [...] > git history shows we tried to remove it once: > > commit 8521fc50d433507a7cdc96bec280f9e5888a54cc > Author: Michal Hocko <mhocko@xxxxxxx> > Date: Tue Jul 26 16:08:29 2011 -0700 > > memcg: get rid of percpu_charge_mutex lock > > but it turned out that the lock did in fact protect a data structure: > the stock itself. Specifically stock->cached: > > commit 9f50fad65b87a8776ae989ca059ad6c17925dfc3 > Author: Michal Hocko <mhocko@xxxxxxx> > Date: Tue Aug 9 11:56:26 2011 +0200 > > Revert "memcg: get rid of percpu_charge_mutex lock" > > This reverts commit 8521fc50d433507a7cdc96bec280f9e5888a54cc. > > The patch incorrectly assumes that using atomic FLUSHING_CACHED_CHARGE > bit operations is sufficient but that is not true. Johannes Weiner has > reported a crash during parallel memory cgroup removal: > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000018 > IP: [<ffffffff81083b70>] css_is_ancestor+0x20/0x70 > Oops: 0000 [#1] PREEMPT SMP > Pid: 19677, comm: rmdir Tainted: G W 3.0.0-mm1-00188-gf38d32b #35 ECS MCP61M-M3/MCP61M-M3 > RIP: 0010:[<ffffffff81083b70>] css_is_ancestor+0x20/0x70 > RSP: 0018:ffff880077b09c88 EFLAGS: 00010202 > Process rmdir (pid: 19677, threadinfo ffff880077b08000, task ffff8800781bb310) > Call Trace: > [<ffffffff810feba3>] mem_cgroup_same_or_subtree+0x33/0x40 > [<ffffffff810feccf>] drain_all_stock+0x11f/0x170 > [<ffffffff81103211>] mem_cgroup_force_empty+0x231/0x6d0 > [<ffffffff811036c4>] mem_cgroup_pre_destroy+0x14/0x20 > [<ffffffff81080559>] cgroup_rmdir+0xb9/0x500 > [<ffffffff81114d26>] vfs_rmdir+0x86/0xe0 > [<ffffffff81114e7b>] do_rmdir+0xfb/0x110 > [<ffffffff81114ea6>] sys_rmdir+0x16/0x20 > [<ffffffff8154d76b>] system_call_fastpath+0x16/0x1b > > We are crashing because we try to dereference cached memcg when we are > checking whether we should wait for draining on the cache. The cache is > already cleaned up, though. > > There is also a theoretical chance that the cached memcg gets freed > between we test for the FLUSHING_CACHED_CHARGE and dereference it in > mem_cgroup_same_or_subtree: > > CPU0 CPU1 CPU2 > mem=stock->cached > stock->cached=NULL > clear_bit > test_and_set_bit > test_bit() ... > <preempted> mem_cgroup_destroy > use after free > > The percpu_charge_mutex protected from this race because sync draining > is exclusive. > > It is safer to revert now and come up with a more parallel > implementation later. > > I didn't remember this one at all! Me neither. Thanks for looking that up! > However, when you look at the codebase from back then, there was no > rcu-protection for memcg lifetime, and drain_stock() didn't double > check stock->cached inside the work. Hence the crash during a race. > > The drain code is different now: drain_local_stock() disables IRQs > which holds up rcu, and then calls drain_stock() and drain_obj_stock() > which both check stock->cached one more time before the deref. > > With workqueue managing concurrency, and rcu ensuring memcg lifetime > during the drain, this lock indeed seems unnecessary now. > > Unless I'm missing something, it should just be removed instead. I do not think you are missing anything. We can drop the lock and simplify the code. The above information would be great to have in the changelog. Thanks! -- Michal Hocko SUSE Labs
