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| author | Oleg Nesterov <oleg@redhat.com> | 2014-09-28 23:44:18 +0200 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2014-10-29 10:07:18 -0700 |
| commit | 392809b25833548ccfc55e61b76c8451a5073216 (patch) | |
| tree | 9cffa1e96d8ecc7208fc3a3c9fc7ef8725d2aa55 /kernel | |
| parent | eca1a08986f622c11b75b3b44d561a1f901c9cec (diff) | |
| download | lwn-392809b25833548ccfc55e61b76c8451a5073216.tar.gz lwn-392809b25833548ccfc55e61b76c8451a5073216.zip | |
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: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/fork.c | 5 | ||||
| -rw-r--r-- | kernel/signal.c | 12 |
2 files changed, 15 insertions, 2 deletions
diff --git a/kernel/fork.c b/kernel/fork.c index 9b7d746d6d62..9ca84189cfc2 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1022,11 +1022,14 @@ void __cleanup_sighand(struct sighand_struct *sighand) { if (atomic_dec_and_test(&sighand->count)) { 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); } } - /* * Initialize POSIX timer handling for a thread group. */ diff --git a/kernel/signal.c b/kernel/signal.c index 54820984a872..19e35135fc60 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1275,7 +1275,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk, local_irq_restore(*flags); 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); if (likely(sighand == tsk->sighand)) { rcu_read_unlock(); |