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Date: Wed, 1 Nov 2017 17:29:33 +0100 From: Christian Brauner <christian.brauner@...onical.com> To: Peter Zijlstra <peterz@...radead.org> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Nikolay Borisov <nborisov@...e.com>, Christian Brauner <christian.brauner@...ntu.com>, Linux Containers <containers@...ts.linux-foundation.org>, tycho@...ho.ws, linux-kernel@...r.kernel.org Subject: Re: [PATCH 3/5] userns: Don't read extents twice in m_start On Wed, Nov 01, 2017 at 03:16:54PM +0100, Peter Zijlstra wrote: > On Wed, Nov 01, 2017 at 03:01:45PM +0100, Christian Brauner wrote: > > Tbf, this isn't solely Eric's fault. I'm to blame here too since I didn't > > document the already existing smb_rmb()s and the new one I introduced when > > writing the patches. I didn't know that there was a hard-set requirement to > > document those. I also didn't see anything in the kernel coding style or the > > memory barriers documentation (But it has been some time since I read those.). > > There's too many documents to read.. I'm not sure we changed > coding-style, and I suspect that'll just end up being another bike-shed > in any case. > > We did get checkpatch changed though, which is a strong enough clue that > something needs to happen. > > But What Nikolay said; memory ordering is hard enough if you're clear on > what exactly you intend to do. But if you later try and reconstruct > without comments, its nearly impossible. Yeah, agreed. I was happy to see that Eric explained his smp_wmb() in detail. That was quite helpful in figuring this out! > > It gets even better if someone changes the ordering requirements over > time and you grow hidden and non-obvious dependencies :/ > > > > Also, you probably want READ_ONCE() here and WRITE_ONCE() in > > > map_write(), the compiler is free to do unordered byte loads/stores > > > without it. > > > > > > And finally, did you want to use smp_store_release() and > > > smp_load_acquire() instead? > > > > Maybe a stupid question but do you suspect this is a real problem in > > this case since you're phrasing it as a question? > > Rhetorical question mostly, I suspect its just what you meant to do, as > per the proposed patch. > > > Iirc, *_acquire() operations include > > locking operations and might come with a greater performance impact then > > smb_{rmb,wmb}(). Given that this is a very performance critical path we should > > be sure. > > No locking what so ever. LOAD-ACQUIRE and STORE-RELEASE are memory ordering > flavours that are paired with the respective memory operation. Ah right, now I remember I was confused by a part of the memory barriers documentation that referenced locks. Acquire operations include locks and smp_load_acquire(). Right, should've remembered that. Thanks! > > It is true that locking ops provide these exact orderings, but that > doesn't imply the reverse. > > In short, store-release is a store that ensures all prior load _and_ > stores happen-before this store. A load-acquire is a load which > happens-before any subsequent load or stores. > > But a release does not constrain later loads or stores, and an acquire > does not constrain prior load or stores. > >
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