Re: CVS commit: src

[maya%NetBSD.org@localhost]

On Tue, Nov 05, 2019 at 09:01:06PM +0000, maya%NetBSD.org@localhost wrote:
> On Tue, Nov 05, 2019 at 08:19:18PM +0000, Maxime Villard wrote:
> > Module Name:	src
> > Committed By:	maxv
> > Date:		Tue Nov  5 20:19:18 UTC 2019
> > 
> > Modified Files:
> > 	src/share/mk: bsd.sys.mk
> > 	src/sys/arch/amd64/amd64: machdep.c mptramp.S
> > 	src/sys/arch/amd64/conf: GENERIC Makefile.amd64
> > 	src/sys/arch/x86/x86: cpu.c
> > 	src/sys/conf: files
> > 	src/sys/kern: files.kern
> > 	src/sys/lib/libkern: libkern.h
> > 	src/sys/sys: atomic.h bus_proto.h cdefs.h systm.h
> > Added Files:
> > 	src/sys/arch/amd64/include: csan.h
> > 	src/sys/kern: subr_csan.c
> > 	src/sys/sys: csan.h
> > 
> > Log Message:
> > Add Kernel Concurrency Sanitizer (kCSan) support. This sanitizer allows us
> > to detect race conditions at runtime. It is a variation of TSan that is
> > easy to implement and more suited to kernel internals, albeit theoretically
> > less precise than TSan's happens-before.
> > 
> > We do basically two things:
> > 
> >  - On every KCSAN_NACCESSES (=2000) memory accesses, we create a cell
> >    describing the access, and delay the calling CPU (10ms).
> > 
> >  - On all memory accesses, we verify if the memory we're reading/writing
> >    is referenced in a cell already.
> > 
> > The combination of the two means that, if for example cpu0 does a read that
> > is selected and cpu1 does a write at the same address, kCSan will fire,
> > because cpu1's write collides with cpu0's read cell.
> > 
> > The coverage of the instrumentation is the same as that of kASan. Also, the
> > code is organized in a way similar to kASan, so it is easy to add support
> > for more architectures than amd64. kCSan is compatible with KCOV.
> > 
> > Reviewed by Kamil.
> > 
> 
> I don't understand how you can distinguish a race from this condition:
> 
> 	CPU0			CPU1
> 
> 
> 				mutex_enter
> 				write (recorded to cell)
> 				mutex_exit
> 
> 	read (checked against record)
> 
> Which is legitimate.^W
would be if I used a mutex on CPU0, but the point still stands. :-)