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proc: fix a dentry lock race between release_task and lookup
[ Upstream commit d919a1e ] Commit 7bc3e6e ("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). Before fix: $ time killall -wq aa real 4m40.946s # During this period, we can see 'ps' and 'systemd' taking high cpu usage. After fix: $ time killall -wq aa real 1m20.732s # During this period, we can see 'systemd' taking high cpu usage. Link: https://lkml.kernel.org/r/[email protected] Fixes: 7bc3e6e ("proc: Use a list of inodes to flush from proc") Link: https://bugzilla.kernel.org/show_bug.cgi?id=216054 Signed-off-by: Zhihao Cheng <[email protected]> Signed-off-by: Zhang Yi <[email protected]> Suggested-by: Brian Foster <[email protected]> Reviewed-by: Brian Foster <[email protected]> Cc: Al Viro <[email protected]> Cc: Alexey Dobriyan <[email protected]> Cc: Eric Biederman <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: Baoquan He <[email protected]> Cc: Kalesh Singh <[email protected]> Cc: Yu Kuai <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Sasha Levin <[email protected]>
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