On Mon, Jun 03, 2019 at 12:01:14PM +0800, Herbert Xu wrote: > On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote: > > > > CPU2: if (b != 1) > > CPU2: b = 1; > > Stop right there. The kernel is full of code that assumes that > assignment to an int/long is atomic. If your compiler breaks this > assumption that we can kiss the kernel good-bye.
The slippery slope apparently started here: : commit ea435467500612636f8f4fb639ff6e76b2496e4b : Author: Matthew Wilcox <matt...@wil.cx> : Date: Tue Jan 6 14:40:39 2009 -0800 : : atomic_t: unify all arch definitions : : diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h : index ad5b9f6ecddf..85b46fba4229 100644 : --- a/arch/x86/include/asm/atomic_32.h : +++ b/arch/x86/include/asm/atomic_32.h : ... : @@ -10,15 +11,6 @@ : * resource counting etc.. : */ : : -/* : - * Make sure gcc doesn't try to be clever and move things around : - * on us. We need to use _exactly_ the address the user gave us, : - * not some alias that contains the same information. : - */ : -typedef struct { : - int counter; : -} atomic_t; : : diff --git a/include/linux/types.h b/include/linux/types.h : index 121f349cb7ec..3b864f2d9560 100644 : --- a/include/linux/types.h : +++ b/include/linux/types.h : @@ -195,6 +195,16 @@ typedef u32 phys_addr_t; : : typedef phys_addr_t resource_size_t; : : +typedef struct { : + volatile int counter; : +} atomic_t; : + Before evolving into the READ_ONCE/WRITE_ONCE that we have now. Linus, are we now really supporting a compiler where an assignment (or a read) from an int/long/pointer can be non-atomic without the volatile marker? Because if that's the case then we have a lot of code to audit. Cheers, -- Email: Herbert Xu <herb...@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt