Re: [PATCH 1/2] irq_work: Fix racy check on work pending flag

[Steven Rostedt]

On Tue, 2012-10-30 at 16:35 +0100, Frederic Weisbecker wrote:
> Work claiming semantics require this operation
> to be SMP-safe.
> 
> So we want a strict ordering between the data we
> want the work to handle and the flags that describe
> the work state such that either we claim and we enqueue
> the work that will see our data or we fail the claim
> but the CPU where the work is still pending will
> see the data we prepared when it executes the work:
> 
> CPU 0                   CPU 1
> 
> data = something        claim work
> smp_mb()                smp_mb()
> try claim work          execute work (data)
> 
> The early check for the pending flag in irq_work_claim()
> fails to meet this ordering requirement. As a result,
> when we fail to claim a work, it may have been processed
> by CPU that were previously owning it already, leaving
> our data unprocessed.
> 
> Discussing this with Steven Rostedt, we can use memory
> barriers to fix this or we can rely on cmpxchg() that
> implies full barrier already.
> 
> To fix this, we start by speculating about the value we
> wish to be in the work->flags but we only make any conclusion
> after the value returned by the cmpxchg() call that either
> claims the work or does the ordering such that the CPU
> where the work is pending handles our data.
> 
> 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>

Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve

> ---
>  kernel/irq_work.c |   22 +++++++++++++++++-----
>  1 files changed, 17 insertions(+), 5 deletions(-)
> 


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