On Wed, Oct 14, 2015 at 06:08:16PM +0200, Dmitry Vyukov wrote: > On Wed, Oct 14, 2015 at 6:01 PM, Paul E. McKenney > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > On Wed, Oct 14, 2015 at 05:50:34PM +0200, Dmitry Vyukov wrote: > >> On Wed, Oct 14, 2015 at 5:45 PM, Paul E. McKenney > >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > >> > On Wed, Oct 14, 2015 at 08:28:43AM -0700, tip-bot for Andrey Ryabinin wrote: > >> >> Commit-ID: 4115ffdf4d6f8986a7abe1dd522c163f599bc0e6 > >> >> Gitweb: http://git.kernel.org/tip/4115ffdf4d6f8986a7abe1dd522c163f599bc0e6 > >> >> Author: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> > >> >> AuthorDate: Tue, 13 Oct 2015 18:28:07 +0300 > >> >> Committer: Ingo Molnar <mingo@xxxxxxxxxx> > >> >> CommitDate: Wed, 14 Oct 2015 16:44:06 +0200 > >> >> > >> >> compiler, atomics: Provide READ_ONCE_NOCHECK() > >> >> > >> >> Some code may perform racy by design memory reads. This could be > >> >> harmless, yet such code may produce KASAN warnings. > >> >> > >> >> To hide such accesses from KASAN this patch introduces > >> >> READ_ONCE_NOCHECK() macro. KASAN will not check the memory > >> >> accessed by READ_ONCE_NOCHECK(). > >> >> > >> >> This patch creates __read_once_size_nocheck() a clone of > >> >> __read_once_size_check() (renamed __read_once_size()). > >> >> The only difference between them is 'no_sanitized_address' > >> >> attribute appended to '*_nocheck' function. This attribute tells > >> >> the compiler that instrumentation of memory accesses should not > >> >> be applied to that function. We declare it as static > >> >> '__maybe_unsed' because GCC is not capable to inline such > >> >> function: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368 > >> >> > >> >> With KASAN=n READ_ONCE_NOCHECK() is just a clone of READ_ONCE(). > >> > > >> > So I add READ_ONCE_NOCHECK() for accesses for which the compiler cannot > >> > prove safe address for KASAN's benefit, but READ_ONCE() suffices for > >> > the data-race-detection logic in KTSAN, correct? > >> > >> KTSAN also needs READ_ONCE_NOCHECK() here. KTSAN will flag races > >> between get_wchan() and the thread accesses to own stack even more > >> aggressively than KASAN, because KTSAN won't like get_wchan() accesses > >> even to non-poisoned areas of other thread stack. > > > > So to keep KTSAN happy, any read from some other thread's stack requires > > READ_ONCE_NOCHECK()? What if the access is via a locking primitive or > > read-modify-write atomic operation? > > > > This is of some interest in RCU, which implements synchronous grace > > periods using completions that are allocated on the calling task's stack > > and manipulated by RCU callbacks that are likely executing elsewhere. > > KTSAN does not have any special logic for stacks. It just generally > flags pairs of accesses when (1) at least one access is not atomic, > (2) at least one access is a write and (3) these accesses are not > synchronized by means of other synchronization. > There is a bunch of cases when kernel code allocates objects on stack > and then passes them to other threads, but as far as there is proper > synchronization it is OK. OK, so let me see if I understand this. ;-) KASAN requires READ_ONCE_NOCHECK() for get_wchan(). KTSAN would be just as happy with READ_ONCE(), but READ_ONCE_NOCHECK() works for both. Did I get it right? Thanx, Paul > For the record, KTSAN is this: > https://github.com/google/ktsan > https://github.com/google/ktsan/wiki/Found-Bugs > -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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