Re: [PATCH] fs/ext4: fix ABBA deadlock in ext4_rmdir()

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (26/08/28 10:48), Baokun Li wrote:
> On 2026/8/27 20:50, Sergey Senozhatsky wrote:
> > We observe the following ABBA deadlock (casefolding enabled):
> >
> >  INFO: task jbd2/dm-13-8:447 blocked for more than 720 seconds.
> >  task:jbd2/dm-13-8    state:D stack:0     pid:447   tgid:447   ppid:2
> >  Call Trace:
> >   <TASK>
> >   __schedule+0xc2c/0xe60
> >   schedule+0x40/0xe0
> >   jbd2_journal_wait_updates+0x8f/0xf0
> >   jbd2_journal_commit_transaction+0x321/0x1850
> >   kjournald2+0xa9/0x230
> >   kthread+0x226/0x2a0
> >   ret_from_fork+0x101/0x1e0
> >   ret_from_fork_asm+0x1a/0x30
> >
> >  INFO: task Thread-10:4161 blocked for more than 720 seconds.
> >  task:Thread-10       state:D stack:0     pid:4161  tgid:3848  ppid:540
> >  Call Trace:
> >   <TASK>
> >   __schedule+0xc2c/0xe60
> >   schedule+0x40/0xe0
> >   wait_transaction_locked+0x8c/0xd0
> >   start_this_handle+0x252/0x870
> >   jbd2__journal_start+0x120/0x280
> >   __ext4_journal_start_sb+0x11a/0x250
> >   ext4_evict_inode+0x208/0x760
> >   iput+0x222/0x5c0
> >   dput+0x293/0x690
> >   ____fput+0x145/0x2b0
> >   task_work_run+0x7a/0xb0
> >   exit_to_user_mode_loop+0xc0/0xd0
> >   do_syscall_64+0x14b/0xf10
> >   entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> >  NMI backtrace for cpu 0
> >  CPU: 0 UID: 1010216 PID: 4160 Comm: Thread-7
> >  RIP: 0010:d_walk+0x0/0x290
> > [..]
> >  Call Trace:
> >  <TASK>
> >   shrink_dcache_parent+0xb2/0x100
> >   d_invalidate+0x50/0x110
> >   ext4_rmdir+0x3a2/0x3d0
> >   vfs_rmdir+0x9d/0x1d0
> >   do_rmdir+0xf0/0x330
> >   __x64_sys_unlinkat+0x34/0x50
> >   do_syscall_64+0x61/0xf10
> >
> > The problem is that ext4_rmdir() calls d_invalidate() before stopping
> > the jbd2 transaction handle.  Because d_invalidate() traverses child
> > dentries it can encounter dying or in-use dentries.  If a concurrent
> > thread executing __dentry_kill() on a child dentry invokes ext4_evict_inode(),
> > it requests a new transaction handle via ext4_journal_start(), which
> > waits for all running handles to close.  However, the thread executing
> > ext4_rmdir() still holds its active transaction handle while looping
> > in shrink_dcache_parent() waiting for the dying child dentry to complete
> > eviction.
> >
> > Stop transaction handle in ext4_rmdir() before calling d_invalidate().
> >
> > Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
> > Co-developed-by: Sarthak Kukreti <sarthakkukreti@google.com>
> > Signed-off-by: Sarthak Kukreti <sarthakkukreti@google.com>
> > Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> 
> Looks good!
> 
> Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>

Thanks!