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authorWill Deacon <will@kernel.org>2019-12-13 14:47:02 +0000
committerWill Deacon <will@kernel.org>2020-04-16 12:28:07 +0100
commit9e343b467c70379e66b8b771d96f03ae23eba351 (patch)
tree7e154d60aa25074d0fefc2c3635093966ec1327b /drivers/xen
parenta5460b5e5fb82656807840d40d3deaecad094044 (diff)
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READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses
{READ,WRITE}_ONCE() cannot guarantee atomicity for arbitrary data sizes. This can be surprising to callers that might incorrectly be expecting atomicity for accesses to aggregate structures, although there are other callers where tearing is actually permissable (e.g. if they are using something akin to sequence locking to protect the access). Linus sayeth: | We could also look at being stricter for the normal READ/WRITE_ONCE(), | and require that they are | | (a) regular integer types | | (b) fit in an atomic word | | We actually did (b) for a while, until we noticed that we do it on | loff_t's etc and relaxed the rules. But maybe we could have a | "non-atomic" version of READ/WRITE_ONCE() that is used for the | questionable cases? The slight snag is that we also have to support 64-bit accesses on 32-bit architectures, as these appear to be widespread and tend to work out ok if either the architecture supports atomic 64-bit accesses (x86, armv7) or if the variable being accesses represents a virtual address and therefore only requires 32-bit atomicity in practice. Take a step in that direction by introducing a variant of 'compiletime_assert_atomic_type()' and use it to check the pointer argument to {READ,WRITE}_ONCE(). Expose __{READ,WRITE}_ONCE() variants which are allowed to tear and convert the one broken caller over to the new macros. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Will Deacon <will@kernel.org>
Diffstat (limited to 'drivers/xen')
-rw-r--r--drivers/xen/time.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index 0968859c29d0..108edbcbc040 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -64,7 +64,7 @@ static void xen_get_runstate_snapshot_cpu_delta(
do {
state_time = get64(&state->state_entry_time);
rmb(); /* Hypervisor might update data. */
- *res = READ_ONCE(*state);
+ *res = __READ_ONCE(*state);
rmb(); /* Hypervisor might update data. */
} while (get64(&state->state_entry_time) != state_time ||
(state_time & XEN_RUNSTATE_UPDATE));