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[Qemu-devel] [PATCH 1/5] include/qemu/atomic.h: Add signal_barrier

From: Richard Henderson
Subject: [Qemu-devel] [PATCH 1/5] include/qemu/atomic.h: Add signal_barrier
Date: Tue, 9 Jul 2019 11:20:45 +0200

We have some potential race conditions vs our user-exec signal
handler that will be solved with this barrier.

Signed-off-by: Richard Henderson <address@hidden>
 include/qemu/atomic.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index a6ac188188..f9cd24c899 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -88,6 +88,13 @@
 #define smp_read_barrier_depends()   barrier()
+ * A signal barrier forces all pending local memory ops to be observed before
+ * a SIGSEGV is delivered to the *same* thread.  In practice this is exactly
+ * the same as barrier(), but since we have the correct builtin, use it.
+ */
+#define signal_barrier()    __atomic_signal_fence(__ATOMIC_SEQ_CST)
 /* Sanity check that the size of an atomic operation isn't "overly large".
  * Despite the fact that e.g. i686 has 64-bit atomic operations, we do not
  * want to use them because we ought not need them, and this lets us do a
@@ -308,6 +315,10 @@
 #define smp_read_barrier_depends()   barrier()
+#ifndef signal_barrier
+#define signal_barrier()    barrier()
 /* These will only be atomic if the processor does the fetch or store
  * in a single issue memory operation

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