From: Zhihao Cheng chengzhihao1@huawei.com

commit d919a1e79bac890421537cf02ae773007bf55e6b upstream.

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).

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/20220713130029.4133533-1-chengzhihao1@huawei.com Fixes: 7bc3e6e55acf06 ("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 chengzhihao1@huawei.com Signed-off-by: Zhang Yi yi.zhang@huawei.com Suggested-by: Brian Foster bfoster@redhat.com Reviewed-by: Brian Foster bfoster@redhat.com Cc: Al Viro viro@zeniv.linux.org.uk Cc: Alexey Dobriyan adobriyan@gmail.com Cc: Eric Biederman ebiederm@xmission.com Cc: Matthew Wilcox willy@infradead.org Cc: Baoquan He bhe@redhat.com Cc: Kalesh Singh kaleshsingh@google.com Cc: Yu Kuai yukuai3@huawei.com Signed-off-by: Andrew Morton akpm@linux-foundation.org [ bp: Context adjustments ] Signed-off-by: Suraj Jitindar Singh surajjs@amazon.com --- fs/proc/base.c | 46 ++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 38 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c index a484c30bd5cf..712948e97991 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -1881,7 +1881,7 @@ void proc_pid_evict_inode(struct proc_inode *ei) put_pid(pid); }

-struct inode *proc_pid_make_inode(struct super_block * sb, +struct inode *proc_pid_make_inode(struct super_block *sb, struct task_struct *task, umode_t mode) { struct inode * inode; @@ -1910,11 +1910,6 @@ struct inode *proc_pid_make_inode(struct super_block * sb,

/* Let the pid remember us for quick removal */ ei->pid = pid; - if (S_ISDIR(mode)) { - spin_lock(&pid->lock); - hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes); - spin_unlock(&pid->lock); - }

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid); security_task_to_inode(task, inode); @@ -1927,6 +1922,39 @@ struct inode *proc_pid_make_inode(struct super_block * sb, return NULL; }

+/* + * Generating an inode and adding it into @pid->inodes, so that task will + * invalidate inode's dentry before being released. + * + * This helper is used for creating dir-type entries under '/proc' and + * '/proc/<tgid>/task'. Other entries(eg. fd, stat) under '/proc/<tgid>' + * can be released by invalidating '/proc/<tgid>' dentry. + * In theory, dentries under '/proc/<tgid>/task' can also be released by + * invalidating '/proc/<tgid>' dentry, we reserve it to handle single + * thread exiting situation: Any one of threads should invalidate its + * '/proc/<tgid>/task/<pid>' dentry before released. + */ +static struct inode *proc_pid_make_base_inode(struct super_block *sb, + struct task_struct *task, umode_t mode) +{ + struct inode *inode; + struct proc_inode *ei; + struct pid *pid; + + inode = proc_pid_make_inode(sb, task, mode); + if (!inode) + return NULL; + + /* Let proc_flush_pid find this directory inode */ + ei = PROC_I(inode); + pid = ei->pid; + spin_lock(&pid->lock); + hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes); + spin_unlock(&pid->lock); + + return inode; +} + int pid_getattr(const struct path *path, struct kstat *stat, u32 request_mask, unsigned int query_flags) { @@ -3341,7 +3369,8 @@ static struct dentry *proc_pid_instantiate(struct dentry * dentry, { struct inode *inode;

- inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO); + inode = proc_pid_make_base_inode(dentry->d_sb, task, + S_IFDIR | S_IRUGO | S_IXUGO); if (!inode) return ERR_PTR(-ENOENT);

@@ -3637,7 +3666,8 @@ static struct dentry *proc_task_instantiate(struct dentry *dentry, struct task_struct *task, const void *ptr) { struct inode *inode; - inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO); + inode = proc_pid_make_base_inode(dentry->d_sb, task, + S_IFDIR | S_IRUGO | S_IXUGO); if (!inode) return ERR_PTR(-ENOENT);