Re: Silencing false lockdep warning related to seq lock
From: Joel Fernandes
Date: Mon May 17 2021 - 21:52:59 EST
Hi Boqun,
Thanks for the reply. See below:
On Mon, May 17, 2021 at 12:21:38PM +0800, Boqun Feng wrote:
[...]
> > After apply Laurent's SPF patchset [1] , we're facing a large number
> > of (seemingly false positive) lockdep reports which are related to
> > circular dependencies with seq locks.
> >
> > lock(A); write_seqcount(B)
> > vs.
> > write_seqcount(B); lock(A)
> >
>
> Two questions here:
>
> * Could you provide the lockdep splats you saw? I wonder whether
> it's similar to the one mentioned in patch #9[1].
Sure I have appended them to this email. Here is the tree with Laurent's
patches applied, in case you want to take a look:
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/refs/heads/chromeos-5.4
Yes, the splat is similar to the one mentioned in that patch #9, however the
first splat I appended below shows an issue with lockdep false positive -
there is clearly problem with lockdep where it thinks following sequence is
bad, when in fact it is not:
init process (skipped some functions):
exec->
flush_old_exec->
exit_mmap ->
free_pgtables->
vm_write_begin(vma) // Y: acquires seq lock in write mode
unlink_anon_vmas // Z: acquires anon_vma->rwsem
exec->
load_elf_binary->
-> setup_arg_pages
-> expand_downwards (in the if (grow <=) block)
mm->page_table_lock // X
vm_write_begin(vma) // Y: acquires seq lock
exec->
do_execve_file->
->get_user_pages
-> handle_pte_fault
-> anon_vma_prepare
-> acquire anon_vma->rwsem // Z
-> acquire mm->page_table_lock // X
If vm_write_begin ever had to wait, then it could lockup like this if following happened concurrently:
Acquire->TryToAcquire
Y->Z
X->Y
Z->X
But Y can never result in a wait since it is a sequence lock. So this is
a lockdep false positive.
>
> * What keeps write_seqcount(vm_seqcount) serialized? If it's only
> one lock that serializes the writers, we probably can make it
> as the nest_lock argument for seqcount_acquire(), and that will
> help prevent the false positives.
I think the write seqcount is serialized by the mmap_sem in all the code
paths I audited in Laurent's patchset.
I am not sure how making it nest_lock argument will help though? The issue is
that lockdep's understanding of seqcount needs to relate seqcount readers to
seqcount writers when considering deadlocks. Unless there's a seqcount reader
involved in the mix, lockdep should just shutup, no? And we don't want it to
shutup incase there is a real locking issue.
Appending splats:
Here is the first splat (which I analuzed above as X, Y, Z):
[ 1.177766] ffff984c36357070 (&anon_vma->rwsem){+.+.}, at: unlink_anon_vmas+0x79/0x1a0
[ 1.179061]
[ 1.179061] but task is already holding lock:
[ 1.180024] ffff984c363520e0 (&vma->vm_sequence){+.+.}, at: exit_mmap+0x96/0x160
[ 1.181207]
[ 1.181207] which lock already depends on the new lock.
[ 1.181207]
[ 1.182543]
[ 1.182543] the existing dependency chain (in reverse order) is:
[ 1.183769]
[ 1.183769] -> #2 (&vma->vm_sequence){+.+.}:
[ 1.184731] expand_downwards+0x21a/0x390
[ 1.185485] setup_arg_pages+0x193/0x220
[ 1.186264] load_elf_binary+0x3d8/0x1570
[ 1.187017] search_binary_handler.part.0+0x56/0x220
[ 1.187942] __do_execve_file+0x680/0xb60
[ 1.188695] do_execve+0x22/0x30
[ 1.189318] call_usermodehelper_exec_async+0x159/0x1b0
[ 1.190259] ret_from_fork+0x3a/0x50
[ 1.190933]
[ 1.190933] -> #1 (&(&mm->page_table_lock)->rlock){+.+.}:
[ 1.192067] _raw_spin_lock+0x27/0x40
[ 1.192763] __anon_vma_prepare+0x5e/0x180
[ 1.193532] handle_pte_fault+0x97c/0x9d0
[ 1.194284] __handle_mm_fault+0x172/0x1f0
[ 1.195049] __get_user_pages+0x322/0x840
[ 1.195804] get_user_pages_remote+0x9f/0x290
[ 1.196609] copy_strings.isra.0+0x1d3/0x360
[ 1.197401] copy_strings_kernel+0x2f/0x40
[ 1.198160] __do_execve_file+0x568/0xb60
[ 1.198915] do_execve+0x22/0x30
[ 1.199547] call_usermodehelper_exec_async+0x159/0x1b0
[ 1.200492] ret_from_fork+0x3a/0x50
[ 1.201173]
[ 1.201173] -> #0 (&anon_vma->rwsem){+.+.}:
[ 1.202104] __lock_acquire+0xe1a/0x1930
[ 1.202837] lock_acquire+0x9a/0x180
[ 1.203520] down_write+0x20/0x60
[ 1.204152] unlink_anon_vmas+0x79/0x1a0
[ 1.204886] free_pgtables+0x89/0x190
[ 1.205575] exit_mmap+0x96/0x160
[ 1.206210] mmput+0x23/0xd0
[ 1.206775] do_exit+0x33b/0xb10
[ 1.207405] do_group_exit+0x34/0xb0
[ 1.208081] __x64_sys_exit_group+0xf/0x10
[ 1.208852] do_syscall_64+0x47/0xb0
[ 1.209529] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 1.210443]
Here are some more splats that could be related:
======================================================
WARNING: possible circular locking dependency detected
5.4.109-lockdep-13764-g4f6fee83851c #1 Tainted: G U W
------------------------------------------------------
chrome/1627 is trying to acquire lock:
ffff8881468c76d8 (&vma->vm_sequence#3){+.+.}, at: do_user_addr_fault+0x9c4/0xba1
ffff8881468e2bd0 (&(&mm->page_table_lock)->rlock){+.+.}, at: expand_downwards+0x50f/0xaaa
_raw_spin_lock+0x3c/0x70
sync_global_pgds_l4+0x127/0x2c3
register_die_notifier+0x17/0x2b
int3_selftest+0x4f/0xca
alternative_instructions+0xf/0x14f
check_bugs+0x155/0x1e5
start_kernel+0x648/0x706
secondary_startup_64+0xa5/0xb0
_raw_spin_lock+0x3c/0x70
pgd_free+0x71/0x1be
__mmdrop+0xbd/0x29d
finish_task_switch+0x527/0x6ad
__schedule+0x117e/0x397d
preempt_schedule_irq+0x94/0xf8
retint_kernel+0x1b/0x2b
__sanitizer_cov_trace_pc+0x0/0x4e
unmap_page_range+0x1427/0x1888
unmap_vmas+0x15f/0x2a0
exit_mmap+0x21b/0x395
__mmput+0xaf/0x2e6
flush_old_exec+0x463/0x640
load_elf_binary+0x5f7/0x1ef1
search_binary_handler+0x17d/0x4fb
load_script+0x759/0x840
search_binary_handler+0x17d/0x4fb
exec_binprm+0x15a/0x549
__do_execve_file+0x8da/0xeef
__x64_sys_execve+0x99/0xa8
do_syscall_64+0x111/0x260
entry_SYSCALL_64_after_hwframe+0x49/0xbe
__lock_acquire+0x25a3/0x5cc0
lock_acquire+0x28a/0x34c
expand_downwards+0x6b0/0xaaa
do_user_addr_fault+0x9c4/0xba1
page_fault+0x34/0x40
Chain exists of:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&(&mm->page_table_lock)->rlock);
lock(pgd_lock);
lock(&(&mm->page_table_lock)->rlock);
lock(&vma->vm_sequence#3);
3 locks held by chrome/1627:
#0: ffff8881468e2c78 (&mm->mmap_sem#2){++++}, at: do_user_addr_fault+0x318/0xba1
#1: ffff888123d64ba0 (&anon_vma->rwsem){++++}, at: expand_downwards+0x1c1/0xaaa
#2: ffff8881468e2bd0 (&(&mm->page_table_lock)->rlock){+.+.}, at: expand_downwards+0x50f/0xaaa
CPU: 1 PID: 1627 Comm: chrome Tainted: G U W 5.4.109-lockdep-13764-g4f6fee83851c #1