Re: [RFC PATCH 1/9] irq_work: Fix racy check on work pending flag

[Steven Rostedt]

On Mon, 2012-10-29 at 14:28 +0100, Frederic Weisbecker wrote:
> Context requirements on irq work claim are not entirely
> clear. But it appears that we can try to claim a work that
> may be already claimed by another CPU.
> 
> If so then the early check on IRQ_WORK_PENDING in
> irq_work_claim() is racy because another CPU may be
> changing the flags concurrently and we have nothing
> to synchronize against that. So the value we deal with
> may be stale for a while already.
> 
> To fix this, start with our best wish as the initial
> value for the work flags and feed cmpxchg with it. But
> only do the check against IRQ_WORK_PENDING flag with the
> cmpxchg result.
> 
> Nonetheless, if the work is not pending but our best wish
> was wrong, restart with the old value returned by cmpxchg.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> ---
>  kernel/irq_work.c |   17 ++++++++++++-----
>  1 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 1588e3b..679c13e 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -34,15 +34,22 @@ static DEFINE_PER_CPU(struct llist_head, irq_work_list);
>   */
>  static bool irq_work_claim(struct irq_work *work)
>  {
> -	unsigned long flags, nflags;
> +	unsigned long flags, oflags, nflags;
>  
> +	/*
> +	 * Can't check IRQ_WORK_PENDING bit right now because the work
> +	 * can be running on another CPU and we are not sync with its
> +	 * changes to work flags. Only cmpxchg can reliably check for us.
> +	 */
> +	flags = work->flags & ~IRQ_WORK_PENDING;
>  	for (;;) {
> -		flags = work->flags;

I wonder if the bug is just a memory barrier missing here? But that also
suggests that the other CPU used a memory barrier too (or cmpxchg()
which implies one).

But this change looks fine too.

-- Steve

> -		if (flags & IRQ_WORK_PENDING)
> -			return false;
>  		nflags = flags | IRQ_WORK_FLAGS;
> -		if (cmpxchg(&work->flags, flags, nflags) == flags)
> +		oflags = cmpxchg(&work->flags, flags, nflags);
> +		if (oflags == flags)
>  			break;
> +		if (oflags & IRQ_WORK_PENDING)
> +			return false;
> +		flags = oflags;
>  		cpu_relax();
>  	}
>  


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