Messages in this thread | | | Date | Wed, 18 May 2016 10:26:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field |
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On Wed, May 18, 2016 at 01:05:55PM +0200, Peter Zijlstra wrote: > On Tue, May 17, 2016 at 12:46:07PM -0700, Paul E. McKenney wrote: > > Actually, if you show a case where this makes a visible system-wide > > difference, you could create a set of primitives for #1 below. Have > > a compiler version check, and if it is an old compiler, map them to > > READ_ONCE() and WRITE_ONCE(), otherwise as follows, though preferably > > with better names: > > > > #define READ_NOTEAR(x) __atomic_load_n(&(x), __ATOMIC_RELAXED) > > #define WRITE_NOTEAR(x, v) __atomic_store_n(&(x), (v), __ATOMIC_RELAXED) > > > > The ambiguity between "no tear" and "not ear" should help motivate a > > better choice of name. > > Alternatively, could we try and talk to our GCC friends to make sure GCC > doesn't tear loads/stores irrespective of what the C language spec > allows?
Interestingly enough, they used to make that guarantee, but removed it when C11 showed up.
Me, I would feel better explicitly telling the compiler what I needed. It is all too easy for bugs to slip in otherwise, especially when the gcc guys are adding exciting new optimizations.
Thanx, Paul
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