On Wed, Oct 14, 2015 at 6:20 PM, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > 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? No, KTSAN also needs READ_ONCE_NOCHECK. READ_ONCE in get_wchan can lead to a data race report. Consider: // the other thead some_stack_var = ...; // get_wchan bp = READ_ONCE(p); // where p happens to point to some_stack_var in the other thread This is generally not atomic and not safe. And this is a data race by all possible definitions. Only READ_ONCE on reading side is not enough to ensure atomicity, also all concurrent writes must be done with atomic operations. -- 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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