Re: scheduling while atomic in z3fold

[Oleksandr Natalenko <oleksandr@natalenko.name>]

On Sat, Nov 28, 2020 at 03:05:24PM +0100, Oleksandr Natalenko wrote:
> Hi.
> 
> While running v5.10-rc5-rt11 I bumped into the following:
> 
> ```
> BUG: scheduling while atomic: git/18695/0x00000002
> Preemption disabled at:
> [<ffffffffbb93fcb3>] z3fold_zpool_malloc+0x463/0x6e0
> …
> Call Trace:
>  dump_stack+0x6d/0x88
>  __schedule_bug.cold+0x88/0x96
>  __schedule+0x69e/0x8c0
>  preempt_schedule_lock+0x51/0x150
>  rt_spin_lock_slowlock_locked+0x117/0x2c0
>  rt_spin_lock_slowlock+0x58/0x80
>  rt_spin_lock+0x2a/0x40
>  z3fold_zpool_malloc+0x4c1/0x6e0
>  zswap_frontswap_store+0x39c/0x980
>  __frontswap_store+0x6e/0xf0
>  swap_writepage+0x39/0x70
>  shmem_writepage+0x31b/0x490
>  pageout+0xf4/0x350
>  shrink_page_list+0xa28/0xcc0
>  shrink_inactive_list+0x300/0x690
>  shrink_lruvec+0x59a/0x770
>  shrink_node+0x2d6/0x8d0
>  do_try_to_free_pages+0xda/0x530
>  try_to_free_pages+0xff/0x260
>  __alloc_pages_slowpath.constprop.0+0x3d5/0x1230
>  __alloc_pages_nodemask+0x2f6/0x350
>  allocate_slab+0x3da/0x660
>  ___slab_alloc+0x4ff/0x760
>  __slab_alloc.constprop.0+0x7a/0x100
>  kmem_cache_alloc+0x27b/0x2c0
>  __d_alloc+0x22/0x230
>  d_alloc_parallel+0x67/0x5e0
>  __lookup_slow+0x5c/0x150
>  path_lookupat+0x2ea/0x4d0
>  filename_lookup+0xbf/0x210
>  vfs_statx.constprop.0+0x4d/0x110
>  __do_sys_newlstat+0x3d/0x80
>  do_syscall_64+0x33/0x40
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> ```
> 
> The preemption seems to be disabled here:
> 
> ```
> $ scripts/faddr2line mm/z3fold.o z3fold_zpool_malloc+0x463
> z3fold_zpool_malloc+0x463/0x6e0:
> add_to_unbuddied at mm/z3fold.c:645
> (inlined by) z3fold_alloc at mm/z3fold.c:1195
> (inlined by) z3fold_zpool_malloc at mm/z3fold.c:1737
> ```
> 
> The call to the rt_spin_lock() seems to be here:
> 
> ```
> $ scripts/faddr2line mm/z3fold.o z3fold_zpool_malloc+0x4c1
> z3fold_zpool_malloc+0x4c1/0x6e0:
> add_to_unbuddied at mm/z3fold.c:649
> (inlined by) z3fold_alloc at mm/z3fold.c:1195
> (inlined by) z3fold_zpool_malloc at mm/z3fold.c:1737
> ```
> 
> Or, in source code:
> 
> ```
>  639 /* Add to the appropriate unbuddied list */
>  640 static inline void add_to_unbuddied(struct z3fold_pool *pool,
>  641                 struct z3fold_header *zhdr)
>  642 {
>  643     if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
>  644             zhdr->middle_chunks == 0) {
>  645         struct list_head *unbuddied = get_cpu_ptr(pool->unbuddied);
>  646
>  647         int freechunks = num_free_chunks(zhdr);
>  648         spin_lock(&pool->lock);
>  649         list_add(&zhdr->buddy, &unbuddied[freechunks]);
>  650         spin_unlock(&pool->lock);
>  651         zhdr->cpu = smp_processor_id();
>  652         put_cpu_ptr(pool->unbuddied);
>  653     }
>  654 }
> ```
> 
> Shouldn't the list manipulation be protected with
> local_lock+this_cpu_ptr instead of get_cpu_ptr+spin_lock?
> 
> Thanks.
> 
> -- 
>   Oleksandr Natalenko (post-factum)

Forgot to Cc linux-rt-users@, sorry.

-- 
  Oleksandr Natalenko (post-factum)