Re: [PATCH RESEND] mm: memcg: synchronize objcg lists with a dedicated spinlock

[Roman Gushchin <guro@fb.com>]

On Wed, Feb 02, 2022 at 09:58:15AM -0600, Jeremy Linton wrote:
> On 2/1/22 16:33, Roman Gushchin wrote:
> > Alexander reported a circular lock dependency revealed by the mmap1
> > ltp test:
> >    LOCKDEP_CIRCULAR (suite: ltp, case: mtest06 (mmap1))
> >            WARNING: possible circular locking dependency detected
> >            5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1 Not tainted
> >            ------------------------------------------------------
> >            mmap1/202299 is trying to acquire lock:
> >            00000001892c0188 (css_set_lock){..-.}-{2:2}, at: obj_cgroup_release+0x4a/0xe0
> >            but task is already holding lock:
> >            00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180
> >            which lock already depends on the new lock.
> >            the existing dependency chain (in reverse order) is:
> >            -> #1 (&sighand->siglock){-.-.}-{2:2}:
> >                   __lock_acquire+0x604/0xbd8
> >                   lock_acquire.part.0+0xe2/0x238
> >                   lock_acquire+0xb0/0x200
> >                   _raw_spin_lock_irqsave+0x6a/0xd8
> >                   __lock_task_sighand+0x90/0x190
> >                   cgroup_freeze_task+0x2e/0x90
> >                   cgroup_migrate_execute+0x11c/0x608
> >                   cgroup_update_dfl_csses+0x246/0x270
> >                   cgroup_subtree_control_write+0x238/0x518
> >                   kernfs_fop_write_iter+0x13e/0x1e0
> >                   new_sync_write+0x100/0x190
> >                   vfs_write+0x22c/0x2d8
> >                   ksys_write+0x6c/0xf8
> >                   __do_syscall+0x1da/0x208
> >                   system_call+0x82/0xb0
> >            -> #0 (css_set_lock){..-.}-{2:2}:
> >                   check_prev_add+0xe0/0xed8
> >                   validate_chain+0x736/0xb20
> >                   __lock_acquire+0x604/0xbd8
> >                   lock_acquire.part.0+0xe2/0x238
> >                   lock_acquire+0xb0/0x200
> >                   _raw_spin_lock_irqsave+0x6a/0xd8
> >                   obj_cgroup_release+0x4a/0xe0
> >                   percpu_ref_put_many.constprop.0+0x150/0x168
> >                   drain_obj_stock+0x94/0xe8
> >                   refill_obj_stock+0x94/0x278
> >                   obj_cgroup_charge+0x164/0x1d8
> >                   kmem_cache_alloc+0xac/0x528
> >                   __sigqueue_alloc+0x150/0x308
> >                   __send_signal+0x260/0x550
> >                   send_signal+0x7e/0x348
> >                   force_sig_info_to_task+0x104/0x180
> >                   force_sig_fault+0x48/0x58
> >                   __do_pgm_check+0x120/0x1f0
> >                   pgm_check_handler+0x11e/0x180
> >            other info that might help us debug this:
> >             Possible unsafe locking scenario:
> >                   CPU0                    CPU1
> >                   ----                    ----
> >              lock(&sighand->siglock);
> >                                           lock(css_set_lock);
> >                                           lock(&sighand->siglock);
> >              lock(css_set_lock);
> >             *** DEADLOCK ***
> >            2 locks held by mmap1/202299:
> >             #0: 00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180
> >             #1: 00000001892ad560 (rcu_read_lock){....}-{1:2}, at: percpu_ref_put_many.constprop.0+0x0/0x168
> >            stack backtrace:
> >            CPU: 15 PID: 202299 Comm: mmap1 Not tainted 5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1
> >            Hardware name: IBM 3906 M04 704 (LPAR)
> >            Call Trace:
> >             [<00000001888aacfe>] dump_stack_lvl+0x76/0x98
> >             [<0000000187c6d7be>] check_noncircular+0x136/0x158
> >             [<0000000187c6e888>] check_prev_add+0xe0/0xed8
> >             [<0000000187c6fdb6>] validate_chain+0x736/0xb20
> >             [<0000000187c71e54>] __lock_acquire+0x604/0xbd8
> >             [<0000000187c7301a>] lock_acquire.part.0+0xe2/0x238
> >             [<0000000187c73220>] lock_acquire+0xb0/0x200
> >             [<00000001888bf9aa>] _raw_spin_lock_irqsave+0x6a/0xd8
> >             [<0000000187ef6862>] obj_cgroup_release+0x4a/0xe0
> >             [<0000000187ef6498>] percpu_ref_put_many.constprop.0+0x150/0x168
> >             [<0000000187ef9674>] drain_obj_stock+0x94/0xe8
> >             [<0000000187efa464>] refill_obj_stock+0x94/0x278
> >             [<0000000187eff55c>] obj_cgroup_charge+0x164/0x1d8
> >             [<0000000187ed8aa4>] kmem_cache_alloc+0xac/0x528
> >             [<0000000187bf2eb8>] __sigqueue_alloc+0x150/0x308
> >             [<0000000187bf4210>] __send_signal+0x260/0x550
> >             [<0000000187bf5f06>] send_signal+0x7e/0x348
> >             [<0000000187bf7274>] force_sig_info_to_task+0x104/0x180
> >             [<0000000187bf7758>] force_sig_fault+0x48/0x58
> >             [<00000001888ae160>] __do_pgm_check+0x120/0x1f0
> >             [<00000001888c0cde>] pgm_check_handler+0x11e/0x180
> >            INFO: lockdep is turned off.
> > 
> > In this example a slab allocation from __send_signal() caused a
> > refilling and draining of a percpu objcg stock, resulted in a
> > releasing of another non-related objcg. Objcg release path requires
> > taking the css_set_lock, which is used to synchronize objcg lists.
> > 
> > This can create a circular dependency with the sighandler lock,
> > which is taken with the locked css_set_lock by the freezer code
> > (to freeze a task).
> > 
> > In general it seems that using css_set_lock to synchronize objcg lists
> > makes any slab allocations and deallocation with the locked
> > css_set_lock and any intervened locks risky.
> > 
> > To fix the problem and make the code more robust let's stop using
> > css_set_lock to synchronize objcg lists and use a new dedicated
> > spinlock instead.
> > 
> > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
> > Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
> > Reviewed-by: Waiman Long <longman@redhat.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: Jeremy Linton <jeremy.linton@arm.com>
> > Cc: cgroups@vger.kernel.org
> > ---
> >   include/linux/memcontrol.h |  5 +++--
> >   mm/memcontrol.c            | 10 +++++-----
> >   2 files changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index b72d75141e12..0abbd685703b 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -219,7 +219,7 @@ struct obj_cgroup {
> >   	struct mem_cgroup *memcg;
> >   	atomic_t nr_charged_bytes;
> >   	union {
> > -		struct list_head list;
> > +		struct list_head list; /* protected by objcg_lock */
> >   		struct rcu_head rcu;
> >   	};
> >   };
> > @@ -315,7 +315,8 @@ struct mem_cgroup {
> >   #ifdef CONFIG_MEMCG_KMEM
> >   	int kmemcg_id;
> >   	struct obj_cgroup __rcu *objcg;
> > -	struct list_head objcg_list; /* list of inherited objcgs */
> > +	/* list of inherited objcgs, protected by objcg_lock */
> > +	struct list_head objcg_list;
> >   #endif
> >   	MEMCG_PADDING(_pad2_);
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 09d342c7cbd0..36e9f38c919d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -254,7 +254,7 @@ struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr)
> >   }
> >   #ifdef CONFIG_MEMCG_KMEM
> > -extern spinlock_t css_set_lock;
> > +static DEFINE_SPINLOCK(objcg_lock);
> >   bool mem_cgroup_kmem_disabled(void)
> >   {
> > @@ -298,9 +298,9 @@ static void obj_cgroup_release(struct percpu_ref *ref)
> >   	if (nr_pages)
> >   		obj_cgroup_uncharge_pages(objcg, nr_pages);
> > -	spin_lock_irqsave(&css_set_lock, flags);
> > +	spin_lock_irqsave(&objcg_lock, flags);
> >   	list_del(&objcg->list);
> > -	spin_unlock_irqrestore(&css_set_lock, flags);
> > +	spin_unlock_irqrestore(&objcg_lock, flags);
> >   	percpu_ref_exit(ref);
> >   	kfree_rcu(objcg, rcu);
> > @@ -332,7 +332,7 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
> >   	objcg = rcu_replace_pointer(memcg->objcg, NULL, true);
> > -	spin_lock_irq(&css_set_lock);
> > +	spin_lock_irq(&objcg_lock);
> >   	/* 1) Ready to reparent active objcg. */
> >   	list_add(&objcg->list, &memcg->objcg_list);
> > @@ -342,7 +342,7 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
> >   	/* 3) Move already reparented objcgs to the parent's list */
> >   	list_splice(&memcg->objcg_list, &parent->objcg_list);
> > -	spin_unlock_irq(&css_set_lock);
> > +	spin_unlock_irq(&objcg_lock);
> >   	percpu_ref_kill(&objcg->refcnt);
> >   }
> > 
> 
> Thanks for taking care of this. Since it looks the same as my patch aside
> from the fact that I also defensivly converted the list_del to a
> list_del_rcu.
> 
> 
> Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
> 
> and
> 
> Tested-by: Jeremy Linton <jeremy.linton@arm.com>
> 

Thank you!