Messages in this thread | | | Date | Fri, 21 Feb 2014 11:53:46 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK |
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Hi Tejun,
On 02/21/2014 08:06 AM, Tejun Heo wrote: > Hello, > > On Fri, Feb 21, 2014 at 07:51:48AM -0500, Peter Hurley wrote: >> I think the vast majority of kernel code which uses the workqueue >> assumes there is a memory ordering guarantee. > > Not really. Workqueues haven't even guaranteed non-reentrancy until > recently, forcing everybody to lock explicitly in the work function. > I don't think there'd be many, if any, use cases which depend on > ordering guarantee on duplicate queueing.
I added some in 3.12 :)
>> Another way to look at this problem is that process_one_work() >> doesn't become the existing instance _until_ PENDING is cleared. > > Sure, having that guarantee definitely is nicer and all we need seems > to be mb_after_unlock in the execution path. Please feel free to > submit a patch.
Ok, I can do that. But AFAIK it'll have to be an smp_rmb(); there is no mb__after unlock.
[ After thinking about it some, I don't think preventing speculative writes before clearing PENDING if useful or necessary, so that's why I'm suggesting only the rmb. ]
>>> add such guarantee, not sure how much it'd matter but it's not like >>> it's gonna cost a lot either. >>> >>> This doesn't have much to do with the current series tho. In fact, >>> PREPARE_WORK can't ever be made to give such guarantee. >> >> Yes, I agree that PREPARE_DELAYED_WORK was also broken usage with the >> same problem. [And there are other bugs in that firewire device work >> code which I'm working on.] >> >>> The function pointer has to fetched before clearing of PENDING. >> >> Why? >> >> As long as the load takes place within the pool->lock, I don't think >> it matters (especially now PREPARE_WORK is removed). > > Hmmm... I was talking about PREPARE_WORK(). Clearing PENDING means > that the work item is released from the worker context and may be > freed or reused at any time (hmm... this may not be true anymore as > non-syncing variants of cancel_work are gone), so clearing PENDING > should be the last access to the work item and thus we can't use that > as the barrier event for fetching its work function.
Yeah, it seems like the work item lifetime is at least guaranteed while either PENDING is set _or_ while the pool->lock is held after PENDING is cleared.
Regards, Peter Hurley
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