Commit 392809b2 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Paul E. McKenney

signal: Document the RCU protection of ->sighand

__cleanup_sighand() frees sighand without RCU grace period. This is
correct but this looks "obviously buggy" and constantly confuses the
readers, add the comments to explain how this works.
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Reviewed-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarPranith Kumar <bobby.prani@gmail.com>
parent eca1a089
...@@ -1022,11 +1022,14 @@ void __cleanup_sighand(struct sighand_struct *sighand) ...@@ -1022,11 +1022,14 @@ void __cleanup_sighand(struct sighand_struct *sighand)
{ {
if (atomic_dec_and_test(&sighand->count)) { if (atomic_dec_and_test(&sighand->count)) {
signalfd_cleanup(sighand); signalfd_cleanup(sighand);
/*
* sighand_cachep is SLAB_DESTROY_BY_RCU so we can free it
* without an RCU grace period, see __lock_task_sighand().
*/
kmem_cache_free(sighand_cachep, sighand); kmem_cache_free(sighand_cachep, sighand);
} }
} }
/* /*
* Initialize POSIX timer handling for a thread group. * Initialize POSIX timer handling for a thread group.
*/ */
......
...@@ -1275,7 +1275,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, ...@@ -1275,7 +1275,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
local_irq_restore(*flags); local_irq_restore(*flags);
break; break;
} }
/*
* This sighand can be already freed and even reused, but
* we rely on SLAB_DESTROY_BY_RCU and sighand_ctor() which
* initializes ->siglock: this slab can't go away, it has
* the same object type, ->siglock can't be reinitialized.
*
* We need to ensure that tsk->sighand is still the same
* after we take the lock, we can race with de_thread() or
* __exit_signal(). In the latter case the next iteration
* must see ->sighand == NULL.
*/
spin_lock(&sighand->siglock); spin_lock(&sighand->siglock);
if (likely(sighand == tsk->sighand)) { if (likely(sighand == tsk->sighand)) {
rcu_read_unlock(); rcu_read_unlock();
......
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