[Devel] Re: [PATCH] c/r: fix race of prepare_descendant() with an ongoing fork()

[Oren Laadan]

Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl at librato.com):
>> From: Oren Laadan <orenl at librato.edu>
>>
>> If prepare_descendants() is walking a tree and one of the tasks is
>> forking, one of two bads can happen. If the child doesn't inherit the
>> ->ctx, it breaks the assumption that the entire subtree is prepared.
>> If the child inherits the ->ctx, it will have one without having taken
>> a reference.
>>
>> This patch closed this race by explicitly getting and referencing the
>> ->ctx for a child process should the parent have one, atomically under
>> the tasklist_lock.
>>
>> Signed-off-by: Oren Laadan <orenl at cs.columbia.edu>
>> ---
>>  kernel/fork.c |   11 ++++++++---
>>  1 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 9f13d7b..57118e4 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -62,6 +62,7 @@
>>  #include <linux/fs_struct.h>
>>  #include <linux/magic.h>
>>  #include <linux/perf_counter.h>
>> +#include <linux/checkpoint.h>
>>
>>  #include <asm/pgtable.h>
>>  #include <asm/pgalloc.h>
>> @@ -1148,9 +1149,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>  	INIT_LIST_HEAD(&p->pi_state_list);
>>  	p->pi_state_cache = NULL;
>>  #endif
>> -#ifdef CONFIG_CHECKPOINT
>> -	p->checkpoint_ctx = NULL;
>> -#endif
>>  	/*
>>  	 * sigaltstack should be cleared when sharing the same VM
>>  	 */
>> @@ -1188,6 +1186,13 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>  	/* Need tasklist lock for parent etc handling! */
>>  	write_lock_irq(&tasklist_lock);
>>
>> +#ifdef CONFIG_CHECKPOINT
>> +	/* If parent is restarting, child should be too */
>> +	if (unlikely(current->checkpoint_ctx)) {
>> +		p->checkpoint_ctx = current->checkpoint_ctx;
> 
> Won't break anything, but technically p->checkpoint_ctx will
> already be copied from current->checkpoint_ctx, so only the
> ckpt_ctx_get() is necessary, so this could really read
> 
> 	if (p->checkpoint_ctx)
> 		ckpt_ctx_get(p->checkpoint_ctx);
> 
> Right?

That's what I initially thought. However, it _is_ possible that
the following occurs:

1) Task A starts a fork an does some of copy_process(): so
@p is NULL, but copy_process() isn't complete

2) An ancestor of task A calls prepare_descendants(), which
then places a valid checkpoint_ctx on A.

That is why I placed this test-and-set snippet while holding
tasklist_lock (write): it ensure that the child has whatever
the parent has, because prepare_descendants() uses the same
lock.

Oren.

> 
>> +		ckpt_ctx_get(p->checkpoint_ctx);
>> +	}
>> +#endif
>>  	/*
>>  	 * The task hasn't been attached yet, so its cpus_allowed mask will
>>  	 * not be changed, nor will its assigned CPU.
>> -- 
>> 1.6.0.4
>>
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