diff options
| -rw-r--r-- | Documentation/memory-barriers.txt | 54 | ||||
| -rw-r--r-- | include/asm-generic/atomic-long.h | 1 | ||||
| -rw-r--r-- | include/linux/atomic.h | 18 | ||||
| -rw-r--r-- | include/linux/compiler.h | 16 | ||||
| -rw-r--r-- | kernel/events/ring_buffer.c | 2 |
5 files changed, 23 insertions, 68 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index b5fe7657456e..aef9487303d0 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -617,16 +617,16 @@ case what's actually required is: However, stores are not speculated. This means that ordering -is- provided for load-store control dependencies, as in the following example: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); if (q) { WRITE_ONCE(b, p); } Control dependencies pair normally with other types of barriers. That -said, please note that READ_ONCE_CTRL() is not optional! Without the -READ_ONCE_CTRL(), the compiler might combine the load from 'a' with -other loads from 'a', and the store to 'b' with other stores to 'b', -with possible highly counterintuitive effects on ordering. +said, please note that READ_ONCE() is not optional! Without the +READ_ONCE(), the compiler might combine the load from 'a' with other +loads from 'a', and the store to 'b' with other stores to 'b', with +possible highly counterintuitive effects on ordering. Worse yet, if the compiler is able to prove (say) that the value of variable 'a' is always non-zero, it would be well within its rights @@ -636,16 +636,12 @@ as follows: q = a; b = p; /* BUG: Compiler and CPU can both reorder!!! */ -Finally, the READ_ONCE_CTRL() includes an smp_read_barrier_depends() -that DEC Alpha needs in order to respect control depedencies. Alternatively -use one of atomic{,64}_read_ctrl(). - -So don't leave out the READ_ONCE_CTRL(). +So don't leave out the READ_ONCE(). It is tempting to try to enforce ordering on identical stores on both branches of the "if" statement as follows: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); if (q) { barrier(); WRITE_ONCE(b, p); @@ -659,7 +655,7 @@ branches of the "if" statement as follows: Unfortunately, current compilers will transform this as follows at high optimization levels: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); barrier(); WRITE_ONCE(b, p); /* BUG: No ordering vs. load from a!!! */ if (q) { @@ -689,7 +685,7 @@ memory barriers, for example, smp_store_release(): In contrast, without explicit memory barriers, two-legged-if control ordering is guaranteed only when the stores differ, for example: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); if (q) { WRITE_ONCE(b, p); do_something(); @@ -698,14 +694,14 @@ ordering is guaranteed only when the stores differ, for example: do_something_else(); } -The initial READ_ONCE_CTRL() is still required to prevent the compiler -from proving the value of 'a'. +The initial READ_ONCE() is still required to prevent the compiler from +proving the value of 'a'. In addition, you need to be careful what you do with the local variable 'q', otherwise the compiler might be able to guess the value and again remove the needed conditional. For example: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); if (q % MAX) { WRITE_ONCE(b, p); do_something(); @@ -718,7 +714,7 @@ If MAX is defined to be 1, then the compiler knows that (q % MAX) is equal to zero, in which case the compiler is within its rights to transform the above code into the following: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); WRITE_ONCE(b, p); do_something_else(); @@ -729,7 +725,7 @@ is gone, and the barrier won't bring it back. Therefore, if you are relying on this ordering, you should make sure that MAX is greater than one, perhaps as follows: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */ if (q % MAX) { WRITE_ONCE(b, p); @@ -746,7 +742,7 @@ of the 'if' statement. You must also be careful not to rely too much on boolean short-circuit evaluation. Consider this example: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); if (q || 1 > 0) WRITE_ONCE(b, 1); @@ -754,7 +750,7 @@ Because the first condition cannot fault and the second condition is always true, the compiler can transform this example as following, defeating control dependency: - q = READ_ONCE_CTRL(a); + q = READ_ONCE(a); WRITE_ONCE(b, 1); This example underscores the need to ensure that the compiler cannot @@ -768,7 +764,7 @@ x and y both being zero: CPU 0 CPU 1 ======================= ======================= - r1 = READ_ONCE_CTRL(x); r2 = READ_ONCE_CTRL(y); + r1 = READ_ONCE(x); r2 = READ_ONCE(y); if (r1 > 0) if (r2 > 0) WRITE_ONCE(y, 1); WRITE_ONCE(x, 1); @@ -797,11 +793,6 @@ site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html. In summary: - (*) Control dependencies must be headed by READ_ONCE_CTRL(), - atomic{,64}_read_ctrl(). Or, as a much less preferable alternative, - interpose smp_read_barrier_depends() between a READ_ONCE() and the - control-dependent write. - (*) Control dependencies can order prior loads against later stores. However, they do -not- guarantee any other sort of ordering: Not prior loads against later loads, nor prior stores against @@ -817,14 +808,13 @@ In summary: between the prior load and the subsequent store, and this conditional must involve the prior load. If the compiler is able to optimize the conditional away, it will have also optimized - away the ordering. Careful use of READ_ONCE_CTRL() READ_ONCE(), - and WRITE_ONCE() can help to preserve the needed conditional. + away the ordering. Careful use of READ_ONCE() and WRITE_ONCE() + can help to preserve the needed conditional. (*) Control dependencies require that the compiler avoid reordering the - dependency into nonexistence. Careful use of READ_ONCE_CTRL(), - atomic{,64}_read_ctrl() or smp_read_barrier_depends() can help to - preserve your control dependency. Please see the Compiler Barrier - section for more information. + dependency into nonexistence. Careful use of READ_ONCE() or + atomic{,64}_read() can help to preserve your control dependency. + Please see the Compiler Barrier section for more information. (*) Control dependencies pair normally with other types of barriers. diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h index f91093c2984c..eb1973bad80b 100644 --- a/include/asm-generic/atomic-long.h +++ b/include/asm-generic/atomic-long.h @@ -43,7 +43,6 @@ static inline long atomic_long_read##mo(const atomic_long_t *l) \ } ATOMIC_LONG_READ_OP() ATOMIC_LONG_READ_OP(_acquire) -ATOMIC_LONG_READ_OP(_ctrl) #undef ATOMIC_LONG_READ_OP diff --git a/include/linux/atomic.h b/include/linux/atomic.h index 27e580d232ca..301de78d65f7 100644 --- a/include/linux/atomic.h +++ b/include/linux/atomic.h @@ -4,15 +4,6 @@ #include <asm/atomic.h> #include <asm/barrier.h> -#ifndef atomic_read_ctrl -static inline int atomic_read_ctrl(const atomic_t *v) -{ - int val = atomic_read(v); - smp_read_barrier_depends(); /* Enforce control dependency. */ - return val; -} -#endif - /* * Relaxed variants of xchg, cmpxchg and some atomic operations. * @@ -561,15 +552,6 @@ static inline int atomic_dec_if_positive(atomic_t *v) #include <asm-generic/atomic64.h> #endif -#ifndef atomic64_read_ctrl -static inline long long atomic64_read_ctrl(const atomic64_t *v) -{ - long long val = atomic64_read(v); - smp_read_barrier_depends(); /* Enforce control dependency. */ - return val; -} -#endif - #ifndef atomic64_andnot static inline void atomic64_andnot(long long i, atomic64_t *v) { diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 3d7810341b57..fe817432190c 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -299,22 +299,6 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s __u.__val; \ }) -/** - * READ_ONCE_CTRL - Read a value heading a control dependency - * @x: The value to be read, heading the control dependency - * - * Control dependencies are tricky. See Documentation/memory-barriers.txt - * for important information on how to use them. Note that in many cases, - * use of smp_load_acquire() will be much simpler. Control dependencies - * should be avoided except on the hottest of hotpaths. - */ -#define READ_ONCE_CTRL(x) \ -({ \ - typeof(x) __val = READ_ONCE(x); \ - smp_read_barrier_depends(); /* Enforce control dependency. */ \ - __val; \ -}) - #endif /* __KERNEL__ */ #endif /* __ASSEMBLY__ */ diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 182bc30899d5..b5d1ea79c595 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -141,7 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle, perf_output_get_handle(handle); do { - tail = READ_ONCE_CTRL(rb->user_page->data_tail); + tail = READ_ONCE(rb->user_page->data_tail); offset = head = local_read(&rb->head); if (!rb->overwrite && unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size)) |
