The pid_revalidate() function requires dropping from RCU into REF lookup
mode. When many threads are resolving paths within /proc in parallel,
this can result in heavy spinlock contention as each thread tries to
grab a reference to the /proc dentry (and drop it shortly thereafter).
Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
updates must be made to the inode due to the owning task performing
setuid(), drop out of RCU and into REF mode.
Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
I'd like to use this patch as an RFC on this approach for reducing spinlock
contention during many parallel path lookups in the /proc filesystem. The
contention can be triggered by, for example, running ~100 parallel instances of
"TZ=/etc/localtime ps -fe >/dev/null" on a 100CPU machine. The %sys utilization
in such a case reaches around 90%, and profiles show two code paths with high
utilization:
walk_component
lookup_fast
unlazy_child
legitimize_root
__legitimize_path
lockref_get_not_dead
terminate_walk
dput
dput
By applying this patch, %sys utilization falls to around 60% under the same
workload.
One item I'd like to highlight about this patch is that the
security_task_to_inode() hook is called less frequently as a result. I don't
know whether this is a major concern, which is why I've included security
reviewers as well.
fs/proc/base.c | 50 ++++++++++++++++++++++++++++++++-------------
fs/proc/internal.h | 5 +++++
include/linux/pid.h | 2 ++
kernel/pid.c | 12 +++++++++++
4 files changed, 55 insertions(+), 14 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebea9501afb8..038056f94ed0 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1813,12 +1813,29 @@ int pid_getattr(const struct path *path, struct kstat *stat,
/*
* Set <pid>/... inode ownership (can change due to setuid(), etc.)
*/
-void pid_update_inode(struct task_struct *task, struct inode *inode)
+static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
+ unsigned int flags)
{
- task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
+ kuid_t uid;
+ kgid_t gid;
+
+ task_dump_owner(task, inode->i_mode, &uid, &gid);
+ if (uid_eq(uid, inode->i_uid) && gid_eq(gid, inode->i_gid) &&
+ !(inode->i_mode & (S_ISUID | S_ISGID)))
+ return 1;
+ if (flags & LOOKUP_RCU)
+ return -ECHILD;
+ inode->i_uid = uid;
+ inode->i_gid = gid;
inode->i_mode &= ~(S_ISUID | S_ISGID);
security_task_to_inode(task, inode);
+ return 1;
+}
+
+void pid_update_inode(struct task_struct *task, struct inode *inode)
+{
+ do_pid_update_inode(task, inode, 0);
}
/*
@@ -1830,19 +1847,24 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
{
struct inode *inode;
struct task_struct *task;
-
- if (flags & LOOKUP_RCU)
- return -ECHILD;
-
- inode = d_inode(dentry);
- task = get_proc_task(inode);
-
- if (task) {
- pid_update_inode(task, inode);
- put_task_struct(task);
- return 1;
+ int rv = 0;
+
+ if (flags & LOOKUP_RCU) {
+ inode = d_inode_rcu(dentry);
+ task = get_proc_task_rcu(inode);
+ if (task) {
+ rv = do_pid_update_inode(task, inode, flags);
+ put_task_struct_rcu_user(task);
+ }
+ } else {
+ inode = d_inode(dentry);
+ task = get_proc_task(inode);
+ if (task) {
+ rv = do_pid_update_inode(task, inode, flags);
+ put_task_struct(task);
+ }
}
- return 0;
+ return rv;
}
static inline bool proc_inode_is_dead(struct inode *inode)
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index cd0c8d5ce9a1..aa6df65ad3eb 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -121,6 +121,11 @@ static inline struct task_struct *get_proc_task(const struct inode *inode)
return get_pid_task(proc_pid(inode), PIDTYPE_PID);
}
+static inline struct task_struct *get_proc_task_rcu(const struct inode *inode)
+{
+ return get_pid_task_rcu_user(proc_pid(inode), PIDTYPE_PID);
+}
+
void task_dump_owner(struct task_struct *task, umode_t mode,
kuid_t *ruid, kgid_t *rgid);
diff --git a/include/linux/pid.h b/include/linux/pid.h
index 9645b1194c98..0b2c54f85e6d 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -86,6 +86,8 @@ static inline struct pid *get_pid(struct pid *pid)
extern void put_pid(struct pid *pid);
extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
+extern struct task_struct *get_pid_task_rcu_user(struct pid *pid,
+ enum pid_type type);
extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
diff --git a/kernel/pid.c b/kernel/pid.c
index 0a9f2e437217..05acbd15cfa6 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -390,6 +390,18 @@ struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
}
EXPORT_SYMBOL_GPL(get_pid_task);
+struct task_struct *get_pid_task_rcu_user(struct pid *pid, enum pid_type type)
+{
+ struct task_struct *task;
+
+ task = pid_task(pid, type);
+ if (task && refcount_inc_not_zero(&task->rcu_users))
+ return task;
+
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(get_pid_task_rcu_user);
+
struct pid *find_get_pid(pid_t nr)
{
struct pid *pid;
--
2.25.1
