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SubjectRe: [RFC,PATCH] Use RCU to protect tasklist for unicast signals
On Fri, Aug 12, 2005 at 12:51:17PM +0400, Oleg Nesterov wrote:
> Paul E. McKenney wrote:
> >
> > --- linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/fs/exec.c 2005-08-11 11:44:55.000000000 -0700
> > +++ linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/fs/exec.c 2005-08-11 12:26:45.000000000 -0700
> > [ ... snip ... ]
> > @@ -785,11 +787,13 @@ no_thread_group:
> > recalc_sigpending();
> >
> > + oldsighand->deleted = 1;
> > + oldsighand->successor = newsighand;
>
> I don't think this is correct.
>
> This ->oldsighand can be shared with another CLONE_SIGHAND
> process and will not be deleted, just unshared.
>
> When the signal is sent to that process we must use ->oldsighand
> for locking, not the oldsighand->successor == newsighand.

OK, the attached instead revalidates that the task struct still references
the sighand_struct after obtaining the lock (and passes kernbench and
LTP, which tells me I need to get better tests!). However, your quick
responses motivated me to read more of the code (yes, I know, I should
have done -that- beforehand!!!), and there are some remaining problems
that I need to sort out, including:

o Fatal signals affect all threads in a process, and the
existing code looks like it assumes that tasklist_lock
is held during delivery of such signals. In this patch,
that assumption does not hold.

o Some of the functions invoked by __group_send_sig_info(),
including handle_stop_signal(), momentarily drop ->siglock.

I will be looking at a few approaches to handle these situations, but,
in the meantime, any other combinations that I missed?

Thanx, Paul


diff -urpNa -X dontdiff linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/fs/exec.c linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/fs/exec.c
--- linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/fs/exec.c 2005-08-11 11:44:55.000000000 -0700
+++ linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/fs/exec.c 2005-08-12 21:18:33.000000000 -0700
@@ -790,7 +790,7 @@ no_thread_group:
write_unlock_irq(&tasklist_lock);

if (atomic_dec_and_test(&oldsighand->count))
- kmem_cache_free(sighand_cachep, oldsighand);
+ sighand_free(oldsighand);
}

BUG_ON(!thread_group_empty(current));
diff -urpNa -X dontdiff linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/include/linux/sched.h linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/include/linux/sched.h
--- linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/include/linux/sched.h 2005-08-11 11:44:57.000000000 -0700
+++ linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/include/linux/sched.h 2005-08-12 21:19:34.000000000 -0700
@@ -450,8 +450,16 @@ struct sighand_struct {
atomic_t count;
struct k_sigaction action[_NSIG];
spinlock_t siglock;
+ struct rcu_head rcu;
};

+static inline void sighand_free(struct sighand_struct *sp)
+{
+ extern void sighand_free_cb(struct rcu_head *rhp);
+
+ call_rcu(&sp->rcu, sighand_free_cb);
+}
+
/*
* NOTE! "signal_struct" does not have it's own
* locking, because a shared signal_struct always
diff -urpNa -X dontdiff linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/kernel/fork.c linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/kernel/fork.c
--- linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/kernel/fork.c 2005-08-11 11:44:57.000000000 -0700
+++ linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/kernel/fork.c 2005-08-12 21:24:00.000000000 -0700
@@ -43,6 +43,7 @@
#include <linux/acct.h>
#include <linux/kthread.h>
#include <linux/notifier.h>
+#include <linux/rcupdate.h>

#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -769,6 +770,14 @@ int unshare_files(void)

EXPORT_SYMBOL(unshare_files);

+void sighand_free_cb(struct rcu_head *rhp)
+{
+ struct sighand_struct *sp =
+ container_of(rhp, struct sighand_struct, rcu);
+
+ kmem_cache_free(sighand_cachep, sp);
+}
+
static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)
{
struct sighand_struct *sig;
diff -urpNa -X dontdiff linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/kernel/signal.c linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/kernel/signal.c
--- linux-2.6.13-rc4-realtime-preempt-V0.7.53-01/kernel/signal.c 2005-08-11 11:44:57.000000000 -0700
+++ linux-2.6.13-rc4-realtime-preempt-V0.7.53-01-tasklistRCU/kernel/signal.c 2005-08-14 19:53:28.000000000 -0700
@@ -328,9 +328,11 @@ void __exit_sighand(struct task_struct *
struct sighand_struct * sighand = tsk->sighand;

/* Ok, we're done with the signal handlers */
+ spin_lock(&sighand->siglock);
tsk->sighand = NULL;
if (atomic_dec_and_test(&sighand->count))
- kmem_cache_free(sighand_cachep, sighand);
+ sighand_free(sighand);
+ spin_unlock(&sighand->siglock);
}

void exit_sighand(struct task_struct *tsk)
@@ -1150,16 +1152,23 @@ void zap_other_threads(struct task_struc
int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
{
unsigned long flags;
+ struct sighand_struct *sp;
int ret;

+retry:
ret = check_kill_permission(sig, info, p);
- if (!ret && sig && p->sighand) {
+ if (!ret && sig && (sp = p->sighand)) {
if (!get_task_struct_rcu(p)) {
return -ESRCH;
}
- spin_lock_irqsave(&p->sighand->siglock, flags);
+ spin_lock_irqsave(&sp->siglock, flags);
+ if (p->sighand != sp) {
+ spin_unlock_irqrestore(&sp->siglock, flags);
+ put_task_struct(p);
+ goto retry;
+ }
ret = __group_send_sig_info(sig, info, p);
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ spin_unlock_irqrestore(&sp->siglock, flags);
put_task_struct(p);
}

-
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