[PATCH] proc: add locking checks in proc_inode_is_dead

[Wen Yang]

The proc_inode_is_dead function might race with __unhash_process.
This will result in a whole bunch of stale proc entries being cached.
To prevent that, add the required locking.

Signed-off-by: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Christian Brauner <christian@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/proc/base.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 1bc9bcd..59720bc 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1994,7 +1994,13 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 
 static inline bool proc_inode_is_dead(struct inode *inode)
 {
-	return !proc_pid(inode)->tasks[PIDTYPE_PID].first;
+	bool has_task;
+
+	read_lock(&tasklist_lock);
+	has_task = pid_has_task(proc_pid(inode), PIDTYPE_PID);
+	read_unlock(&tasklist_lock);
+
+	return !has_task;
 }
 
 int pid_delete_dentry(const struct dentry *dentry)
-- 
1.8.3.1