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From e7d0e29d1962092af58d0445439671a6e1d91f71 Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 9 Mar 2023 08:10:33 -0500
Subject: [PATCH 02/13] qatomic: add smp_mb__before/after_rmw()
RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw()
RH-Bugzilla: 2168472
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: Eric Auger <eric.auger@redhat.com>
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Commit: [2/10] 1f87eb3157abcf23f020881cedce42f76497f348
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472
commit ff00bed1897c3d27adc5b0cec6f6eeb5a7d13176
Author: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu Mar 2 11:10:56 2023 +0100
qatomic: add smp_mb__before/after_rmw()
On ARM, seqcst loads and stores (which QEMU does not use) are compiled
respectively as LDAR and STLR instructions. Even though LDAR is
also used for load-acquire operations, it also waits for all STLRs to
leave the store buffer. Thus, LDAR and STLR alone are load-acquire
and store-release operations, but LDAR also provides store-against-load
ordering as long as the previous store is a STLR.
Compare this to ARMv7, where store-release is DMB+STR and load-acquire
is LDR+DMB, but an additional DMB is needed between store-seqcst and
load-seqcst (e.g. DMB+STR+DMB+LDR+DMB); or with x86, where MOV provides
load-acquire and store-release semantics and the two can be reordered.
Likewise, on ARM sequentially consistent read-modify-write operations only
need to use LDAXR and STLXR respectively for the load and the store, while
on x86 they need to use the stronger LOCK prefix.
In a strange twist of events, however, the _stronger_ semantics
of the ARM instructions can end up causing bugs on ARM, not on x86.
The problems occur when seqcst atomics are mixed with relaxed atomics.
QEMU's atomics try to bridge the Linux API (that most of the developers
are familiar with) and the C11 API, and the two have a substantial
difference:
- in Linux, strongly-ordered atomics such as atomic_add_return() affect
the global ordering of _all_ memory operations, including for example
READ_ONCE()/WRITE_ONCE()
- in C11, sequentially consistent atomics (except for seq-cst fences)
only affect the ordering of sequentially consistent operations.
In particular, since relaxed loads are done with LDR on ARM, they are
not ordered against seqcst stores (which are done with STLR).
QEMU implements high-level synchronization primitives with the idea that
the primitives contain the necessary memory barriers, and the callers can
use relaxed atomics (qatomic_read/qatomic_set) or even regular accesses.
This is very much incompatible with the C11 view that seqcst accesses
are only ordered against other seqcst accesses, and requires using seqcst
fences as in the following example:
qatomic_set(&y, 1); qatomic_set(&x, 1);
smp_mb(); smp_mb();
... qatomic_read(&x) ... ... qatomic_read(&y) ...
When a qatomic_*() read-modify write operation is used instead of one
or both stores, developers that are more familiar with the Linux API may
be tempted to omit the smp_mb(), which will work on x86 but not on ARM.
This nasty difference between Linux and C11 read-modify-write operations
has already caused issues in util/async.c and more are being found.
Provide something similar to Linux smp_mb__before/after_atomic(); this
has the double function of documenting clearly why there is a memory
barrier, and avoiding a double barrier on x86 and s390x systems.
The new macro can already be put to use in qatomic_mb_set().
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
docs/devel/atomics.rst | 26 +++++++++++++++++++++-----
include/qemu/atomic.h | 17 ++++++++++++++++-
2 files changed, 37 insertions(+), 6 deletions(-)
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst
index 52baa0736d..10fbfc58bb 100644
--- a/docs/devel/atomics.rst
+++ b/docs/devel/atomics.rst
@@ -25,7 +25,8 @@ provides macros that fall in three camps:
- weak atomic access and manual memory barriers: ``qatomic_read()``,
``qatomic_set()``, ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``,
- ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``;
+ ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``,
+ ``smp_mb__before_rmw()``, ``smp_mb__after_rmw()``;
- sequentially consistent atomic access: everything else.
@@ -470,7 +471,7 @@ and memory barriers, and the equivalents in QEMU:
sequential consistency.
- in QEMU, ``qatomic_read()`` and ``qatomic_set()`` do not participate in
- the total ordering enforced by sequentially-consistent operations.
+ the ordering enforced by read-modify-write operations.
This is because QEMU uses the C11 memory model. The following example
is correct in Linux but not in QEMU:
@@ -486,9 +487,24 @@ and memory barriers, and the equivalents in QEMU:
because the read of ``y`` can be moved (by either the processor or the
compiler) before the write of ``x``.
- Fixing this requires an ``smp_mb()`` memory barrier between the write
- of ``x`` and the read of ``y``. In the common case where only one thread
- writes ``x``, it is also possible to write it like this:
+ Fixing this requires a full memory barrier between the write of ``x`` and
+ the read of ``y``. QEMU provides ``smp_mb__before_rmw()`` and
+ ``smp_mb__after_rmw()``; they act both as an optimization,
+ avoiding the memory barrier on processors where it is unnecessary,
+ and as a clarification of this corner case of the C11 memory model:
+
+ +--------------------------------+
+ | QEMU (correct) |
+ +================================+
+ | :: |
+ | |
+ | a = qatomic_fetch_add(&x, 2);|
+ | smp_mb__after_rmw(); |
+ | b = qatomic_read(&y); |
+ +--------------------------------+
+
+ In the common case where only one thread writes ``x``, it is also possible
+ to write it like this:
+--------------------------------+
| QEMU (correct) |
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 112a29910b..7855443cab 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -243,6 +243,20 @@
#define smp_wmb() smp_mb_release()
#define smp_rmb() smp_mb_acquire()
+/*
+ * SEQ_CST is weaker than the older __sync_* builtins and Linux
+ * kernel read-modify-write atomics. Provide a macro to obtain
+ * the same semantics.
+ */
+#if !defined(QEMU_SANITIZE_THREAD) && \
+ (defined(__i386__) || defined(__x86_64__) || defined(__s390x__))
+# define smp_mb__before_rmw() signal_barrier()
+# define smp_mb__after_rmw() signal_barrier()
+#else
+# define smp_mb__before_rmw() smp_mb()
+# define smp_mb__after_rmw() smp_mb()
+#endif
+
/* qatomic_mb_read/set semantics map Java volatile variables. They are
* less expensive on some platforms (notably POWER) than fully
* sequentially consistent operations.
@@ -257,7 +271,8 @@
#if !defined(__SANITIZE_THREAD__) && \
(defined(__i386__) || defined(__x86_64__) || defined(__s390x__))
/* This is more efficient than a store plus a fence. */
-# define qatomic_mb_set(ptr, i) ((void)qatomic_xchg(ptr, i))
+# define qatomic_mb_set(ptr, i) \
+ ({ (void)qatomic_xchg(ptr, i); smp_mb__after_rmw(); })
#else
# define qatomic_mb_set(ptr, i) \
({ qatomic_store_release(ptr, i); smp_mb(); })
--
2.37.3
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