On Mon, May 13, 2024 at 08:41:27AM -0700, Paul E. McKenney wrote: [...] > > > +#include <linux/types.h> > > > +#include <linux/export.h> > > > +#include <linux/instrumented.h> > > > +#include <linux/atomic.h> > > > +#include <linux/panic.h> > > > +#include <linux/bug.h> > > > +#include <asm-generic/rwonce.h> > > > +#include <linux/cmpxchg-emu.h> > > > + > > > +union u8_32 { > > > + u8 b[4]; > > > + u32 w; > > > +}; > > > + > > > +/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */ > > > +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new) > > > +{ > > > + u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3); > > > + int i = ((uintptr_t)p) & 0x3; > > > + union u8_32 old32; > > > + union u8_32 new32; > > > + u32 ret; > > > + > > > + ret = READ_ONCE(*p32); > > > + do { > > > + old32.w = ret; > > > + if (old32.b[i] != old) > > > + return old32.b[i]; > > > + new32.w = old32.w; > > > + new32.b[i] = new; > > > + instrument_atomic_read_write(p, 1); > > > + ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above. > > > > Just out of curiosity, why is this `data_race` needed? cmpxchg is atomic > > so there should be no chance for a data race? > > That is what I thought, too. ;-) > > The problem is that the cmpxchg() covers 32 bits, and so without that > data_race(), KCSAN would complain about data races with perfectly > legitimate concurrent accesses to the other three bytes. > > The instrument_atomic_read_write(p, 1) beforehand tells KCSAN to complain > about concurrent accesses, but only to that one byte. > Oh, I see. For that purpose, maybe we can just use raw_cmpxchg() here, i.e. a cmpxchg() without any instrument in it. Cc Mark in case I'm missing something. Regards, Boqun > Thanx, Paul >