[tip: locking/kcsan] fault_inject: Don't rely on "return value" from WRITE_ONCE()

[tip-bot2 for Will Deacon]

The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID:     9a7cb2d8d6b959fc11a34668b1523f745ae5f714
Gitweb:        https://git.kernel.org/tip/9a7cb2d8d6b959fc11a34668b1523f745ae5f714
Author:        Will Deacon <will@xxxxxxxxxx>
AuthorDate:    Mon, 11 May 2020 21:41:40 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 12 May 2020 11:04:12 +02:00

fault_inject: Don't rely on "return value" from WRITE_ONCE()

It's a bit weird that WRITE_ONCE() evaluates to the value it stores and
it's also different to smp_store_release(), which can't be used this
way.

In preparation for preventing this in WRITE_ONCE(), change the fault
injection code to use a local variable instead.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Akinobu Mita <akinobu.mita@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20200511204150.27858-9-will@xxxxxxxxxx

---
 lib/fault-inject.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/fault-inject.c b/lib/fault-inject.c
index 8186ca8..ce12621 100644
--- a/lib/fault-inject.c
+++ b/lib/fault-inject.c
@@ -106,7 +106,9 @@ bool should_fail(struct fault_attr *attr, ssize_t size)
 		unsigned int fail_nth = READ_ONCE(current->fail_nth);
 
 		if (fail_nth) {
-			if (!WRITE_ONCE(current->fail_nth, fail_nth - 1))
+			fail_nth--;
+			WRITE_ONCE(current->fail_nth, fail_nth);
+			if (!fail_nth)
 				goto fail;
 
 			return false;