Hello!
This patch is an experiment in use of RCU for individual code paths that
read-acquire the tasklist lock, in this case, unicast signal delivery.
It passes five kernbenches on 4-CPU x86, but obviously needs much more
testing before it is considered for serious use, let alone inclusion.
My main question is whether I have the POSIX semantics covered. I believe
that I do, but thought I should check with people who are more familiar
with POSIX than am I.
For the record, some shortcomings of this patch:
o Needs lots more testing on more architectures.
o Needs performance and stress testing.
o Needs testing in Ingo's PREEMPT_RT environment.
o Uses cmpxchg(), which is currently architecture dependent.
This can be fixed, for example, by using the hashed locks
proposed in an earlier patch from Dipankar:
http://marc.theaimsgroup.com/?l=linux-kernel&m=111875978502912&w=2
Thoughts?
Thanx, Paul
---
Not-signed-off-by: [email protected]
include/linux/sched.h | 27 +++++++++++++++++++++++++--
kernel/sched.c | 5 +++++
kernel/signal.c | 8 ++++++--
3 files changed, 36 insertions(+), 4 deletions(-)
diff -urpN -X dontdiff linux-2.6.13-rc6/include/linux/sched.h linux-2.6.13-rc6-tasklistRCU/include/linux/sched.h
--- linux-2.6.13-rc6/include/linux/sched.h 2005-08-08 19:59:23.000000000 -0700
+++ linux-2.6.13-rc6-tasklistRCU/include/linux/sched.h 2005-08-09 15:44:48.000000000 -0700
@@ -34,6 +34,7 @@
#include <linux/percpu.h>
#include <linux/topology.h>
#include <linux/seccomp.h>
+#include <linux/rcupdate.h>
struct exec_domain;
@@ -770,6 +771,7 @@ struct task_struct {
int cpuset_mems_generation;
#endif
atomic_t fs_excl; /* holding fs exclusive resources */
+ struct rcu_head rcu;
};
static inline pid_t process_group(struct task_struct *tsk)
@@ -793,8 +795,29 @@ static inline int pid_alive(struct task_
extern void free_task(struct task_struct *tsk);
extern void __put_task_struct(struct task_struct *tsk);
#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)
-#define put_task_struct(tsk) \
-do { if (atomic_dec_and_test(&(tsk)->usage)) __put_task_struct(tsk); } while(0)
+
+static inline int get_task_struct_rcu(struct task_struct *t)
+{
+ int oldusage;
+
+ do {
+ oldusage = atomic_read(&t->usage);
+ if (oldusage == 0) {
+ return 0;
+ }
+ } while (cmpxchg(&t->usage.counter,
+ oldusage, oldusage + 1) != oldusage);
+ return 1;
+}
+
+extern void __put_task_struct_cb(struct rcu_head *rhp);
+
+static inline void put_task_struct(struct task_struct *t)
+{
+ if (atomic_dec_and_test(&t->usage)) {
+ call_rcu(&t->rcu, __put_task_struct_cb);
+ }
+}
/*
* Per process flags
diff -urpN -X dontdiff linux-2.6.13-rc6/kernel/sched.c linux-2.6.13-rc6-tasklistRCU/kernel/sched.c
--- linux-2.6.13-rc6/kernel/sched.c 2005-08-08 19:59:24.000000000 -0700
+++ linux-2.6.13-rc6-tasklistRCU/kernel/sched.c 2005-08-09 12:27:34.000000000 -0700
@@ -176,6 +176,11 @@ static unsigned int task_timeslice(task_
#define task_hot(p, now, sd) ((long long) ((now) - (p)->last_ran) \
< (long long) (sd)->cache_hot_time)
+void __put_task_struct_cb(struct rcu_head *rhp)
+{
+ __put_task_struct(container_of(rhp, struct task_struct, rcu));
+}
+
/*
* These are the runqueue data structures:
*/
diff -urpN -X dontdiff linux-2.6.13-rc6/kernel/signal.c linux-2.6.13-rc6-tasklistRCU/kernel/signal.c
--- linux-2.6.13-rc6/kernel/signal.c 2005-08-08 19:59:24.000000000 -0700
+++ linux-2.6.13-rc6-tasklistRCU/kernel/signal.c 2005-08-10 08:20:25.000000000 -0700
@@ -1151,9 +1151,13 @@ int group_send_sig_info(int sig, struct
ret = check_kill_permission(sig, info, p);
if (!ret && sig && p->sighand) {
+ if (!get_task_struct_rcu(p)) {
+ return -ESRCH;
+ }
spin_lock_irqsave(&p->sighand->siglock, flags);
ret = __group_send_sig_info(sig, info, p);
spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ put_task_struct(p);
}
return ret;
@@ -1200,12 +1204,12 @@ kill_proc_info(int sig, struct siginfo *
int error;
struct task_struct *p;
- read_lock(&tasklist_lock);
+ rcu_read_lock();
p = find_task_by_pid(pid);
error = -ESRCH;
if (p)
error = group_send_sig_info(sig, info, p);
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
return error;
}
-
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