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Message-ID: <20220713130029.4133533-1-chengzhihao1@huawei.com>
Date: Wed, 13 Jul 2022 21:00:29 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: <ebiederm@...ssion.com>, <willy@...radead.org>, <bhe@...hat.com>,
<bfoster@...hat.com>, <akpm@...ux-foundation.org>,
<kaleshsingh@...gle.com>
CC: <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
<chengzhihao1@...wei.com>, <yukuai3@...wei.com>
Subject: [PATCH v5] proc: Fix a dentry lock race between release_task and lookup
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.
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@...wei.com>
Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
Suggested-by: Brian Foster <bfoster@...hat.com>
---
v1->v2: Add new helper proc_pid_make_base_inode that performs the extra
work of adding to the pid->list.
v2->v3: Add performance regression in commit message.
v3->v4: Make proc_pid_make_base_inode() static
v4->v5: Add notes to explain what proc_pid_make_base_inode() does
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 8dfa36a99c74..93f7e3d971e4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1885,7 +1885,7 @@ 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;
@@ -1914,11 +1914,6 @@ 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);
@@ -1931,6 +1926,39 @@ 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(struct user_namespace *mnt_userns, const struct path *path,
struct kstat *stat, u32 request_mask, unsigned int query_flags)
{
@@ -3369,7 +3397,8 @@ 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);
@@ -3671,7 +3700,8 @@ 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);
--
2.31.1
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