[PATCH v2] fork: fix oops after fork failure

[Glauber Costa <glommer@parallels.com>]

When we want to duplicate a new process, dup_task_struct() will undergo
a series of allocations. If alloc_thread_info_node() fails, we call
free_task_struct() and return.

This seems right, but it is not. free_task_struct() will not only free
the task struct from the kmem_cache, but will also call
arch_release_task_struct(). The problem is that this function is
supposed to undo whatever arch-specific work done by
arch_dup_task_struct(), that is not yet called at this point.  The
particular problem I ran accross was that in x86, we will arrive at
fpu_free() without having ever allocated it.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Reported-by: Frederic Weisbecker <fweisbec@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Tony Luck <tony.luck@intel.com>
CC: Fenghua Yu <fenghua.yu@intel.com>
---
 arch/ia64/include/asm/thread_info.h | 2 +-
 kernel/fork.c                       | 9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/ia64/include/asm/thread_info.h b/arch/ia64/include/asm/thread_info.h
index f7ee853..62452bd 100644
--- a/arch/ia64/include/asm/thread_info.h
+++ b/arch/ia64/include/asm/thread_info.h
@@ -90,7 +90,7 @@ struct thread_info {
 										\
 	ret;									\
 })
-#define free_task_struct(tsk)	free_pages((unsigned long) (tsk), KERNEL_STACK_SIZE_ORDER)
+#define __free_task_struct(tsk)	free_pages((unsigned long) (tsk), KERNEL_STACK_SIZE_ORDER)
 
 #endif /* !__ASSEMBLY */
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 152d023..76acb1a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -124,10 +124,15 @@ static inline struct task_struct *alloc_task_struct_node(int node)
 
 void __weak arch_release_task_struct(struct task_struct *tsk) { }
 
+static inline void __free_task_struct(struct task_struct *tsk)
+{
+	kmem_cache_free(task_struct_cachep, tsk);
+}
+
 static inline void free_task_struct(struct task_struct *tsk)
 {
 	arch_release_task_struct(tsk);
-	kmem_cache_free(task_struct_cachep, tsk);
+	__free_task_struct(tsk);
 }
 #endif
 
@@ -299,7 +304,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 
 	ti = alloc_thread_info_node(tsk, node);
 	if (!ti) {
-		free_task_struct(tsk);
+		__free_task_struct(tsk);
 		return NULL;
 	}
 
-- 
1.7.11.4