On Mon, Jun 15, 2020 at 12:08:30PM -0400, Waiman Long wrote:
> Depending on the workloads, the following circular locking dependency
> warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo
> lock) may show up:
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.0.0-rc1+ #60 Tainted: G W
> ------------------------------------------------------
> fsfreeze/4346 is trying to acquire lock:
> 0000000026f1d784 (fs_reclaim){+.+.}, at:
> fs_reclaim_acquire.part.19+0x5/0x30
>
> but task is already holding lock:
> 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>
> which lock already depends on the new lock.
> :
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(sb_internal);
> lock(fs_reclaim);
> lock(sb_internal);
> lock(fs_reclaim);
>
> *** DEADLOCK ***
>
> 4 locks held by fsfreeze/4346:
> #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650
> #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at:
> freeze_super+0xda/0x290
> #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650
> #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>
> stack backtrace:
> Call Trace:
> dump_stack+0xe0/0x19a
> print_circular_bug.isra.10.cold.34+0x2f4/0x435
> check_prev_add.constprop.19+0xca1/0x15f0
> validate_chain.isra.14+0x11af/0x3b50
> __lock_acquire+0x728/0x1200
> lock_acquire+0x269/0x5a0
> fs_reclaim_acquire.part.19+0x29/0x30
> fs_reclaim_acquire+0x19/0x20
> kmem_cache_alloc+0x3e/0x3f0
> kmem_zone_alloc+0x79/0x150
> xfs_trans_alloc+0xfa/0x9d0
> xfs_sync_sb+0x86/0x170
> xfs_log_sbcount+0x10f/0x140
> xfs_quiesce_attr+0x134/0x270
> xfs_fs_freeze+0x4a/0x70
> freeze_super+0x1af/0x290
> do_vfs_ioctl+0xedc/0x16c0
> ksys_ioctl+0x41/0x80
> __x64_sys_ioctl+0x73/0xa9
> do_syscall_64+0x18f/0xd23
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> This is a false positive as all the dirty pages are flushed out before
> the filesystem can be frozen.
>
> Perhaps breaking the fs_reclaim pseudo lock into a per filesystem lock
> may fix the issue. However, that will greatly complicate the logic and
> may not be worth it.
>
> Another way to fix it is to disable the taking of the fs_reclaim
> pseudo lock when in the freezing code path as a reclaim on the
> freezed filesystem is not possible. By using the newly introduced
> PF_MEMALLOC_NOLOCKDEP flag, lockdep checking is disabled in
> xfs_trans_alloc() if XFS_TRANS_NO_WRITECOUNT flag is set.
>
> In the freezing path, there is another path where memory allocation
> is being done without the XFS_TRANS_NO_WRITECOUNT flag:
>
> xfs_fs_freeze()
> => xfs_quiesce_attr()
> => xfs_log_quiesce()
> => xfs_log_unmount_write()
> => xlog_unmount_write()
> => xfs_log_reserve()
> => xlog_ticket_alloc()
>
> In this case, we just disable fs reclaim for this particular 600 bytes
> memory allocation.
>
> Without this patch, the command sequence below will show that the lock
> dependency chain sb_internal -> fs_reclaim exists.
>
> # fsfreeze -f /home
> # fsfreeze --unfreeze /home
> # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal
>
> After applying the patch, such sb_internal -> fs_reclaim lock dependency
> chain can no longer be found. Because of that, the locking dependency
> warning will not be shown.
>
> Suggested-by: Dave Chinner <[email protected]>
> Signed-off-by: Waiman Long <[email protected]>
> ---
> fs/xfs/xfs_log.c | 9 +++++++++
> fs/xfs/xfs_trans.c | 30 ++++++++++++++++++++++++++----
> 2 files changed, 35 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 00fda2e8e738..33244680d0d4 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -830,8 +830,17 @@ xlog_unmount_write(
> xfs_lsn_t lsn;
> uint flags = XLOG_UNMOUNT_TRANS;
> int error;
> + unsigned long pflags;
>
> + /*
> + * xfs_log_reserve() allocates memory. This can lead to fs reclaim
> + * which may conflicts with the unmount process. To avoid that,
> + * disable fs reclaim for this allocation.
> + */
> + current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS);
> error = xfs_log_reserve(mp, 600, 1, &tic, XFS_LOG, 0);
> + current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS);
> +
> if (error)
> goto out_err;
>
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 3c94e5ff4316..ddb10ad3f51f 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -255,7 +255,27 @@ xfs_trans_alloc(
> struct xfs_trans **tpp)
> {
> struct xfs_trans *tp;
> - int error;
> + int error = 0;
> + unsigned long pflags = -1;
> +
> + /*
> + * When XFS_TRANS_NO_WRITECOUNT is set, it means there are no dirty
> + * data pages in the filesystem at this point.
That's not true. Look at the other callers of xfs_trans_alloc_empty.
Also: Why not set PF_MEMALLOC_NOFS at the start of the freeze call
chain?
--D
> + * So even if fs reclaim
> + * is being done, it won't happen to this filesystem. In this case,
> + * PF_MEMALLOC_NOLOCKDEP should be set to avoid false positive
> + * lockdep splat like:
> + *
> + * CPU0 CPU1
> + * ---- ----
> + * lock(sb_internal);
> + * lock(fs_reclaim);
> + * lock(sb_internal);
> + * lock(fs_reclaim);
> + *
> + * *** DEADLOCK ***
> + */
> + if (PF_MEMALLOC_NOLOCKDEP && (flags & XFS_TRANS_NO_WRITECOUNT))
> + current_set_flags_nested(&pflags, PF_MEMALLOC_NOLOCKDEP);
>
> /*
> * Allocate the handle before we do our freeze accounting and setting up
> @@ -284,13 +304,15 @@ xfs_trans_alloc(
> error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> if (error) {
> xfs_trans_cancel(tp);
> - return error;
> + goto out;
> }
>
> trace_xfs_trans_alloc(tp, _RET_IP_);
> -
> *tpp = tp;
> - return 0;
> +out:
> + if (PF_MEMALLOC_NOLOCKDEP && (pflags != -1))
> + current_restore_flags_nested(&pflags, PF_MEMALLOC_NOLOCKDEP);
> + return error;
> }
>
> /*
> --
> 2.18.1
>
