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Date: Fri, 10 Jul 2009 03:48:57 +0200 From: Oleg Nesterov <oleg@...hat.com> To: Christoph Hellwig <hch@...radead.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, David Howells <dhowells@...hat.com>, James Morris <jmorris@...ei.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Roland McGrath <roland@...hat.com>, Stephen Smalley <sds@...ho.nsa.gov>, linux-kernel@...r.kernel.org Subject: [PATCH v2 -mm 2/2] current_is_single_threaded: don't use ->mmap_sem current_is_single_threaded() can safely miss a freshly forked CLONE_VM task, but in this case it must not miss its parent. That is why we take mm->mmap_sem for writing to make sure a thread/task with the same ->mm can't pass exit_mm() and disappear. However we can avoid ->mmap_sem and rely on rcu/barriers: - if we do not see the exiting parent on thread/process list we see the result of list_del_rcu(), in this case we must also see the result of list_add_rcu() which does wmb(). - if we do see the parent but its ->mm == NULL, we need rmb() to make sure we can't miss the child. Signed-off-by: Oleg Nesterov <oleg@...hat.com> --- lib/is_single_threaded.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) --- WAIT/lib/is_single_threaded.c~ISS_2_RCU 2009-07-09 22:16:29.000000000 +0200 +++ WAIT/lib/is_single_threaded.c 2009-07-09 22:54:41.000000000 +0200 @@ -22,8 +22,6 @@ bool current_is_single_threaded(void) struct task_struct *p, *t; bool ret; - might_sleep(); - if (atomic_read(&task->signal->live) != 1) return false; @@ -31,7 +29,6 @@ bool current_is_single_threaded(void) return true; ret = false; - down_write(&mm->mmap_sem); rcu_read_lock(); for_each_process(p) { if (unlikely(p->flags & PF_KTHREAD)) @@ -45,12 +42,17 @@ bool current_is_single_threaded(void) goto found; if (likely(t->mm)) break; + /* + * t->mm == NULL. Make sure next_thread/next_task + * will see other CLONE_VM tasks which might be + * forked before exiting. + */ + smp_rmb(); } while_each_thread(p, t); } ret = true; found: rcu_read_unlock(); - up_write(&mm->mmap_sem); return ret; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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