On Wed, Oct 14, 2015 at 07:23:02PM +0200, Dmitry Vyukov wrote: > On Wed, Oct 14, 2015 at 7:04 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. > > > > OK. However, this is specific to get_wchan()'s out-of-bounds stack, right? > > Yes... and no. > This is specific to racy accesses, not necessary to out-of-bounds > stack. If you have a data race in-bounds, it is also not OK :) But a non-data-race out of bounds would presumably trigger KASAN but not KTSAN. > > If I have multiple tasks accessing some other task's on-stack variable, > > then as long as all other potentially concurrent accesses use atomic > > operations, READ_ONCE() suffices. Or am I still missing something here? > > This is correct. > Generally, the idea is that KTSAN flags what you think is a bug, and > does not flag what you think is not a bug. If you have all proper > synchronization in place, then KTSAN should not flag that; whether the > object is on stack or not is irrelevant. Good! Thanx, Paul -- 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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