[PATCH] signals: Allow tasks to cache one sigqueue struct, version 2

[Joe Korty <joe.korty@concurrent-rt.com>]

[5.4.28-rt19] Cache up to one sigqueue per task.

This replaces an earlier version of this rt patch,

    signals-allow-rt-tasks-to-cache-one-sigqueue-struct.patch

which is able to generate a kernel splat of the form,

    virt_to_cache: Object is not a Slab page!
    ...
    Call Trace:
     free_uid+0x93/0xa0
     __dequeue_signal+0x21c/0x230
     dequeue_signal+0xb7/0x1b0
     get_signal+0x266/0xce0
     do_signal+0x36/0x640

This most easily happens when the test program, tb.c,
is executed.  That test program can be found in an
earlier linux-rt-users posting,

   Kill(2) and pthread_create(2) interact poorly in 5.4.28-rt19
   Date: Tue, 28 Apr 2020 11:45:36 -0400

After poking around the original patch, not making any
progress, I tried revamping the patch down to its simpliest
possible implementation.  This revamped version has been
tested in both an rt and nort compiled 5.4.28-rt19 kernel.
No issues have been seen so far.

Please consider

  1) dropping the original cache-sigqueue patch, or
  2) replace that patch with this new version, or
  3) rewrite the original patch using your own criteria
     for what a correct patch should be.

and backport whatever fix is selected to all versions of the
rt kernel that have the original patch applied.

Thanks,
Joe

---
signals: Allow tasks to cache one sigqueue struct, version 2

To avoid allocation overhead, allow all tasks to cache one sigqueue struct
in task struct.

[ This replaces version 1, which is unstable in several versions of the
rt patch. ]

Signed-off-by: Joe Korty <joe.korty@concurrent-rt.com>

Index: b/include/linux/sched.h
===================================================================
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -935,6 +935,7 @@ struct task_struct {
 	/* Signal handlers: */
 	struct signal_struct		*signal;
 	struct sighand_struct		*sighand;
+	struct sigqueue			*sigqueue_cache;
 	sigset_t			blocked;
 	sigset_t			real_blocked;
 	/* Restored if set_restore_sigmask() was used: */
Index: b/include/linux/signal.h
===================================================================
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -255,6 +255,7 @@ static inline void init_sigpending(struc
 }
 
 extern void flush_sigqueue(struct sigpending *queue);
+extern void flush_task_sigqueue(struct task_struct *tsk);
 
 /* Test if 'sig' is valid signal. Use this instead of testing _NSIG directly */
 static inline int valid_signal(unsigned long sig)
Index: b/kernel/exit.c
===================================================================
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -171,6 +171,7 @@ static void __exit_signal(struct task_st
 		flush_sigqueue(&sig->shared_pending);
 		tty_kref_put(tty);
 	}
+	flush_task_sigqueue(tsk);
 }
 
 static void delayed_put_task_struct(struct rcu_head *rhp)
Index: b/kernel/fork.c
===================================================================
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1587,6 +1587,7 @@ static int copy_signal(unsigned long clo
 	init_waitqueue_head(&sig->wait_chldexit);
 	sig->curr_target = tsk;
 	init_sigpending(&sig->shared_pending);
+	tsk->sigqueue_cache = NULL;
 	INIT_HLIST_HEAD(&sig->multiprocess);
 	seqlock_init(&sig->stats_lock);
 	prev_cputime_init(&sig->prev_cputime);
@@ -1924,6 +1925,7 @@ static __latent_entropy struct task_stru
 	spin_lock_init(&p->alloc_lock);
 
 	init_sigpending(&p->pending);
+	p->sigqueue_cache = NULL;
 
 	p->utime = p->stime = p->gtime = 0;
 #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
Index: b/kernel/signal.c
===================================================================
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -431,7 +431,9 @@ __sigqueue_alloc(int sig, struct task_st
 	rcu_read_unlock();
 
 	if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
-		q = kmem_cache_alloc(sigqueue_cachep, flags);
+		q = xchg(&t->sigqueue_cache, NULL);
+		if (!q)
+			q = kmem_cache_alloc(sigqueue_cachep, flags);
 	} else {
 		print_dropped_signal(sig);
 	}
@@ -454,7 +456,9 @@ static void __sigqueue_free(struct sigqu
 		return;
 	if (atomic_dec_and_test(&q->user->sigpending))
 		free_uid(q->user);
-	kmem_cache_free(sigqueue_cachep, q);
+	q = xchg(&current->sigqueue_cache, q);
+	if (q)
+		kmem_cache_free(sigqueue_cachep, q);
 }
 
 void flush_sigqueue(struct sigpending *queue)
@@ -469,6 +473,15 @@ void flush_sigqueue(struct sigpending *q
 	}
 }
 
+void flush_task_sigqueue(struct task_struct *t)
+{
+	struct sigqueue *q;
+
+	q = xchg(&t->sigqueue_cache, NULL);
+	if (q)
+		kmem_cache_free(sigqueue_cachep, q);
+}
+
 /*
  * Flush all pending signals for this kthread.
  */