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Previous to the change to distinguish read-write accesses, when
CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y is set, KCSAN would consider
the non-atomic bitops as atomic. We want to partially revert to this
behaviour, but with one important distinction: report racing
modifications, since lost bits due to non-atomicity are certainly
possible.
Given the operations here only modify a single bit, assuming
non-atomicity of the writer is sufficient may be reasonable for certain
usage (and follows the permissible nature of the "assume plain writes
atomic" rule). In other words:
1. We want non-atomic read-modify-write races to be reported;
this is accomplished by kcsan_check_read(), where any
concurrent write (atomic or not) will generate a report.
2. We do not want to report races with marked readers, but -do-
want to report races with unmarked readers; this is
accomplished by the instrument_write() ("assume atomic
write" with Kconfig option set).
With the above rules, when KCSAN_ASSUME_PLAIN_WRITES_ATOMIC is selected,
it is hoped that KCSAN's reporting behaviour is better aligned with
current expected permissible usage for non-atomic bitops.
Note that, a side-effect of not telling KCSAN that the accesses are
read-writes, is that this information is not displayed in the access
summary in the report. It is, however, visible in inline-expanded stack
traces. For now, it does not make sense to introduce yet another special
case to KCSAN's runtime, only to cater to the case here.
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Currently bitops-instrumented.h assumes that the architecture provides
atomic, non-atomic and locking bitops (e.g. both set_bit and __set_bit).
This is true on x86 and s390, but is not always true: there is a
generic bitops/non-atomic.h header that provides generic non-atomic
operations, and also a generic bitops/lock.h for locking operations.
powerpc uses the generic non-atomic version, so it does not have it's
own e.g. __set_bit that could be renamed arch___set_bit.
Split up bitops-instrumented.h to mirror the atomic/non-atomic/lock
split. This allows arches to only include the headers where they
have arch-specific versions to rename. Update x86 and s390.
(The generic operations are automatically instrumented because they're
written in C, not asm.)
Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190820024941.12640-1-dja@axtens.net
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