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Filipe Manana 28307d938f percpu: make pcpu_alloc() aware of current gfp context 
Since 5.7-rc1, on btrfs we have a percpu counter initialization for
which we always pass a GFP_KERNEL gfp_t argument (this happens since
commit 2992df7326 ("btrfs: Implement DREW lock")).

That is safe in some contextes but not on others where allowing fs
reclaim could lead to a deadlock because we are either holding some
btrfs lock needed for a transaction commit or holding a btrfs
transaction handle open.  Because of that we surround the call to the
function that initializes the percpu counter with a NOFS context using
memalloc_nofs_save() (this is done at btrfs_init_fs_root()).

However it turns out that this is not enough to prevent a possible
deadlock because percpu_alloc() determines if it is in an atomic context
by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this
case) and it is not aware that a NOFS context is set.

Because percpu_alloc() thinks it is in a non atomic context it locks the
pcpu_alloc_mutex.  This can result in a btrfs deadlock when
pcpu_balance_workfn() is running, has acquired that mutex and is waiting
for reclaim, while the btrfs task that called percpu_counter_init() (and
therefore percpu_alloc()) is holding either the btrfs commit_root
semaphore or a transaction handle (done fs/btrfs/backref.c:
iterate_extent_inodes()), which prevents reclaim from finishing as an
attempt to commit the current btrfs transaction will deadlock.

Lockdep reports this issue with the following trace:

  ======================================================
  WARNING: possible circular locking dependency detected
  5.6.0-rc7-btrfs-next-77 #1 Not tainted
  ------------------------------------------------------
  kswapd0/91 is trying to acquire lock:
  ffff8938a3b3fdc8 (&delayed_node->mutex){+.+.}, at: __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]

  but task is already holding lock:
  ffffffffb4f0dbc0 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x5/0x30

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #4 (fs_reclaim){+.+.}:
         fs_reclaim_acquire.part.0+0x25/0x30
         __kmalloc+0x5f/0x3a0
         pcpu_create_chunk+0x19/0x230
         pcpu_balance_workfn+0x56a/0x680
         process_one_work+0x235/0x5f0
         worker_thread+0x50/0x3b0
         kthread+0x120/0x140
         ret_from_fork+0x3a/0x50

  -> #3 (pcpu_alloc_mutex){+.+.}:
         __mutex_lock+0xa9/0xaf0
         pcpu_alloc+0x480/0x7c0
         __percpu_counter_init+0x50/0xd0
         btrfs_drew_lock_init+0x22/0x70 [btrfs]
         btrfs_get_fs_root+0x29c/0x5c0 [btrfs]
         resolve_indirect_refs+0x120/0xa30 [btrfs]
         find_parent_nodes+0x50b/0xf30 [btrfs]
         btrfs_find_all_leafs+0x60/0xb0 [btrfs]
         iterate_extent_inodes+0x139/0x2f0 [btrfs]
         iterate_inodes_from_logical+0xa1/0xe0 [btrfs]
         btrfs_ioctl_logical_to_ino+0xb4/0x190 [btrfs]
         btrfs_ioctl+0x165a/0x3130 [btrfs]
         ksys_ioctl+0x87/0xc0
         __x64_sys_ioctl+0x16/0x20
         do_syscall_64+0x5c/0x260
         entry_SYSCALL_64_after_hwframe+0x49/0xbe

  -> #2 (&fs_info->commit_root_sem){++++}:
         down_write+0x38/0x70
         btrfs_cache_block_group+0x2ec/0x500 [btrfs]
         find_free_extent+0xc6a/0x1600 [btrfs]
         btrfs_reserve_extent+0x9b/0x180 [btrfs]
         btrfs_alloc_tree_block+0xc1/0x350 [btrfs]
         alloc_tree_block_no_bg_flush+0x4a/0x60 [btrfs]
         __btrfs_cow_block+0x122/0x5a0 [btrfs]
         btrfs_cow_block+0x106/0x240 [btrfs]
         commit_cowonly_roots+0x55/0x310 [btrfs]
         btrfs_commit_transaction+0x509/0xb20 [btrfs]
         sync_filesystem+0x74/0x90
         generic_shutdown_super+0x22/0x100
         kill_anon_super+0x14/0x30
         btrfs_kill_super+0x12/0x20 [btrfs]
         deactivate_locked_super+0x31/0x70
         cleanup_mnt+0x100/0x160
         task_work_run+0x93/0xc0
         exit_to_usermode_loop+0xf9/0x100
         do_syscall_64+0x20d/0x260
         entry_SYSCALL_64_after_hwframe+0x49/0xbe

  -> #1 (&space_info->groups_sem){++++}:
         down_read+0x3c/0x140
         find_free_extent+0xef6/0x1600 [btrfs]
         btrfs_reserve_extent+0x9b/0x180 [btrfs]
         btrfs_alloc_tree_block+0xc1/0x350 [btrfs]
         alloc_tree_block_no_bg_flush+0x4a/0x60 [btrfs]
         __btrfs_cow_block+0x122/0x5a0 [btrfs]
         btrfs_cow_block+0x106/0x240 [btrfs]
         btrfs_search_slot+0x50c/0xd60 [btrfs]
         btrfs_lookup_inode+0x3a/0xc0 [btrfs]
         __btrfs_update_delayed_inode+0x90/0x280 [btrfs]
         __btrfs_commit_inode_delayed_items+0x81f/0x870 [btrfs]
         __btrfs_run_delayed_items+0x8e/0x180 [btrfs]
         btrfs_commit_transaction+0x31b/0xb20 [btrfs]
         iterate_supers+0x87/0xf0
         ksys_sync+0x60/0xb0
         __ia32_sys_sync+0xa/0x10
         do_syscall_64+0x5c/0x260
         entry_SYSCALL_64_after_hwframe+0x49/0xbe

  -> #0 (&delayed_node->mutex){+.+.}:
         __lock_acquire+0xef0/0x1c80
         lock_acquire+0xa2/0x1d0
         __mutex_lock+0xa9/0xaf0
         __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
         btrfs_evict_inode+0x40d/0x560 [btrfs]
         evict+0xd9/0x1c0
         dispose_list+0x48/0x70
         prune_icache_sb+0x54/0x80
         super_cache_scan+0x124/0x1a0
         do_shrink_slab+0x176/0x440
         shrink_slab+0x23a/0x2c0
         shrink_node+0x188/0x6e0
         balance_pgdat+0x31d/0x7f0
         kswapd+0x238/0x550
         kthread+0x120/0x140
         ret_from_fork+0x3a/0x50

  other info that might help us debug this:

  Chain exists of:
    &delayed_node->mutex --> pcpu_alloc_mutex --> fs_reclaim

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(fs_reclaim);
                                 lock(pcpu_alloc_mutex);
                                 lock(fs_reclaim);
    lock(&delayed_node->mutex);

   *** DEADLOCK ***

  3 locks held by kswapd0/91:
   #0: (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x5/0x30
   #1: (shrinker_rwsem){++++}, at: shrink_slab+0x12f/0x2c0
   #2: (&type->s_umount_key#43){++++}, at: trylock_super+0x16/0x50

  stack backtrace:
  CPU: 1 PID: 91 Comm: kswapd0 Not tainted 5.6.0-rc7-btrfs-next-77 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
  Call Trace:
   dump_stack+0x8f/0xd0
   check_noncircular+0x170/0x190
   __lock_acquire+0xef0/0x1c80
   lock_acquire+0xa2/0x1d0
   __mutex_lock+0xa9/0xaf0
   __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
   btrfs_evict_inode+0x40d/0x560 [btrfs]
   evict+0xd9/0x1c0
   dispose_list+0x48/0x70
   prune_icache_sb+0x54/0x80
   super_cache_scan+0x124/0x1a0
   do_shrink_slab+0x176/0x440
   shrink_slab+0x23a/0x2c0
   shrink_node+0x188/0x6e0
   balance_pgdat+0x31d/0x7f0
   kswapd+0x238/0x550
   kthread+0x120/0x140
   ret_from_fork+0x3a/0x50

This could be fixed by making btrfs pass GFP_NOFS instead of GFP_KERNEL
to percpu_counter_init() in contextes where it is not reclaim safe,
however that type of approach is discouraged since
memalloc_[nofs|noio]_save() were introduced.  Therefore this change
makes pcpu_alloc() look up into an existing nofs/noio context before
deciding whether it is in an atomic context or not.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Link: http://lkml.kernel.org/r/20200430164356.15543-1-fdmanana@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-05-07 19:27:21 -07:00
arch arch/x86/kvm/svm/sev.c: change flag passed to GUP fast in sev_pin_memory() 2020-05-07 19:27:20 -07:00
block block: remove the bd_openers checks in blk_drop_partitions 2020-04-30 10:25:43 -06:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2020-04-01 14:47:40 -07:00
Documentation Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
drivers Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
fs eventpoll: fix missing wakeup for ovflist in ep_poll_callback 2020-05-07 19:27:20 -07:00
include Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
init Kbuild updates for v5.7 (2nd) 2020-04-11 09:46:12 -07:00
ipc ipc/mqueue.c: change __do_notify() to bypass check_kill_permission() 2020-05-07 19:27:20 -07:00
kernel kernel/kcov.c: fix typos in kcov_remote_start documentation 2020-05-07 19:27:20 -07:00
lib linux-kselftest-kunit-5.7-rc4 2020-04-30 16:32:47 -07:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm percpu: make pcpu_alloc() aware of current gfp context 2020-05-07 19:27:21 -07:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
samples vmalloc: fix remap_vmalloc_range() bounds checks 2020-04-21 11:11:56 -07:00
scripts scripts/gdb: repair rb_first() and rb_last() 2020-05-07 19:27:20 -07:00
security selinux/stable-5.7 PR 20200430 2020-04-30 16:35:45 -07:00
sound sound fixes for 5.7-rc4 2020-05-01 11:05:28 -07:00
tools Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
usr kbuild: fix comment about missing include guard detection 2020-04-11 12:09:48 +09:00
virt KVM: Pass kvm_init()'s opaque param to additional arch funcs 2020-03-31 10:48:03 -04:00
.clang-format clang-format: Update with the latest for_each macro list 2020-04-18 13:49:33 +02:00
.cocciconfig
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.gitignore .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
.mailmap mailmap: Add Sedat Dilek (replacement for expired email address) 2020-04-11 09:28:34 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Hand MIPS over to Thomas 2020-02-24 22:43:18 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
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MAINTAINERS Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-06 20:53:22 -07:00
Makefile Linux 5.7-rc4 2020-05-03 14:56:04 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.