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SubjectRe: [PATCH v4] proc: Fix a dentry lock race between release_task and lookup
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在 2022/6/1 14:23, Zhihao Cheng 写道:
friendly ping
> Commit 7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc")
> moved proc_flush_task() behind __exit_signal(). Then, process systemd
> can take long period high cpu usage during releasing task in following
> concurrent processes:
>
> systemd ps
> kernel_waitid stat(/proc/tgid)
> do_wait filename_lookup
> wait_consider_task lookup_fast
> release_task
> __exit_signal
> __unhash_process
> detach_pid
> __change_pid // remove task->pid_links
> d_revalidate -> pid_revalidate // 0
> d_invalidate(/proc/tgid)
> shrink_dcache_parent(/proc/tgid)
> d_walk(/proc/tgid)
> spin_lock_nested(/proc/tgid/fd)
> // iterating opened fd
> proc_flush_pid |
> d_invalidate (/proc/tgid/fd) |
> shrink_dcache_parent(/proc/tgid/fd) |
> shrink_dentry_list(subdirs) ↓
> shrink_lock_dentry(/proc/tgid/fd) --> race on dentry lock
>
> Function d_invalidate() will remove dentry from hash firstly, but why does
> proc_flush_pid() process dentry '/proc/tgid/fd' before dentry '/proc/tgid'?
> That's because proc_pid_make_inode() adds proc inode in reverse order by
> invoking hlist_add_head_rcu(). But proc should not add any inodes under
> '/proc/tgid' except '/proc/tgid/task/pid', fix it by adding inode into
> 'pid->inodes' only if the inode is /proc/tgid or /proc/tgid/task/pid.
>
> Performance regression:
> Create 200 tasks, each task open one file for 50,000 times. Kill all
> tasks when opened files exceed 10,000,000 (cat /proc/sys/fs/file-nr).
>

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