Messages in this thread | | | Date | Tue, 27 Mar 2018 15:13:39 +0200 | From | Andrea Parri <> | Subject | Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is_locked() |
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On Tue, Mar 27, 2018 at 10:33:06PM +1100, Benjamin Herrenschmidt wrote: > On Tue, 2018-03-27 at 12:25 +0200, Andrea Parri wrote: > > > I would rather wait until it is properly documented. Debugging that IPC > > > problem took a *LOT* of time and energy, I wouldn't want these issues > > > to come and bite us again. > > > > I understand. And I'm grateful for this debugging as well as for the (IMO) > > excellent account of it you provided in 51d7d5205d338. > > > > Said this ;) I cannot except myself from saying that I would probably have > > resisted that solution (adding an smp_mb() in my arch_spin_is_locked), and > > instead "blamed"/suggested that caller to fix his memory ordering... > > This is debatable. I tend go for the conservative approach assuming > most people using fairly high level APIs such as spin_is_locked() are > not fully cognisant of the subtleties of our memory model. > > After all, it works on x86 ... > > The fact that the load can leak into the internals of spin_lock() > implementation and re-order with the lock word load itself is quite > hard to fathom for somebody who doesn't have years of experience > dealing with that sort of issue. > > So people will get it wrong. > > So unless it's very performance sensitive, I'd rather have things like > spin_is_locked be conservative by default and provide simpler ordering > semantics.
Well, it might not be "very performance sensitive" but allow me to say that "40+ SYNCs in stuff like BUG_ON or such" is sadness to my eyes ;), especially when considered that our "high level API" provides means to avoid this situation (e.g., smp_mb__after_spinlock(); BTW, if you look at architectures for which this macro is "non-trivial", you can get an idea of the architectures which "wouldn't work"; of course, x86 is not among these). Yes, we do appear to have different views on what is to be considered the "simpler ordering semantics". I'm willing to change mine _as soon as_ this gets documented: would you be willing to send a patch (on the lines of my [1]) to describe/document such semantics?
Andrea
> > Cheers, > Ben. >
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