Revision d936eb23874433caa3e3d841cfa16f5434b85dcf authored by Linus Torvalds on 16 July 2021, 01:05:31 UTC, committed by Linus Torvalds on 16 July 2021, 01:05:31 UTC
This reverts commit b7eb335e26a9c7f258c96b3962c283c379d3ede0.
It turns out that the problem with the clang -Wimplicit-fallthrough
warning is not about the kernel source code, but about clang itself, and
that the warning is unusable until clang fixes its broken ways.
In particular, when you enable this warning for clang, you not only get
warnings about implicit fallthroughs. You also get this:
warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
which is completely broken becasue it
(a) doesn't even tell you where the problem is (seriously: no line
numbers, no filename, no nothing).
(b) is fundamentally broken anyway, because there are perfectly valid
reasons to have a fallthrough statement even if it turns out that
it can perhaps not be reached.
In the kernel, an example of that second case is code in the scheduler:
switch (state) {
case cpuset:
if (IS_ENABLED(CONFIG_CPUSETS)) {
cpuset_cpus_allowed_fallback(p);
state = possible;
break;
}
fallthrough;
case possible:
where if CONFIG_CPUSETS is enabled you actually never hit the
fallthrough case at all. But that in no way makes the fallthrough
wrong.
So the warning is completely broken, and enabling it for clang is a very
bad idea.
In the meantime, we can keep the gcc option enabled, and make the gcc
build use
-Wimplicit-fallthrough=5
which means that we will at least continue to require a proper
fallthrough statement, and that gcc won't silently accept the magic
comment versions. Because gcc does this all correctly, and while the odd
"=5" part is kind of obscure, it's documented in [1]:
"-Wimplicit-fallthrough=5 doesn’t recognize any comments as
fallthrough comments, only attributes disable the warning"
so if clang ever fixes its bad behavior we can try enabling it there again.
Link: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html [1]
Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 1013d4a
atomic_bitops.txt
=============
Atomic bitops
=============
While our bitmap_{}() functions are non-atomic, we have a number of operations
operating on single bits in a bitmap that are atomic.
API
---
The single bit operations are:
Non-RMW ops:
test_bit()
RMW atomic operations without return value:
{set,clear,change}_bit()
clear_bit_unlock()
RMW atomic operations with return value:
test_and_{set,clear,change}_bit()
test_and_set_bit_lock()
Barriers:
smp_mb__{before,after}_atomic()
All RMW atomic operations have a '__' prefixed variant which is non-atomic.
SEMANTICS
---------
Non-atomic ops:
In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
RMW ops:
The test_and_{}_bit() operations return the original value of the bit.
ORDERING
--------
Like with atomic_t, the rule of thumb is:
- non-RMW operations are unordered;
- RMW operations that have no return value are unordered;
- RMW operations that have a return value are fully ordered.
- RMW operations that are conditional are unordered on FAILURE,
otherwise the above rules apply. In the case of test_and_{}_bit() operations,
if the bit in memory is unchanged by the operation then it is deemed to have
failed.
Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
clear_bit_unlock() which has RELEASE semantics.
Since a platform only has a single means of achieving atomic operations
the same barriers as for atomic_t are used, see atomic_t.txt.
Computing file changes ...
