On 20.09.22 12:29, Vlastimil Babka wrote:
On 9/20/22 11:17, Alexander Atanasov wrote:
Hello,
On 20.09.22 11:42, Vlastimil Babka wrote:
+static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
+ size_t length)
+{
+ if (s->refcount > 1)
+ return -EINVAL;
+
+ s->flags &= ~SLAB_FAILSLAB;
+ if (buf[0] == '1')
+ s->flags |= SLAB_FAILSLAB;
Could we at least use a temporary variable to set up the final value and
then do a WRITE_ONCE() to s->flags, so the compiler is not allowed to do
some funky stuff? Assuming this is really the only place where we modify
s->flags during runtime, so we can't miss other updates due to RMW.
Since it is set or clear - instead of temporary variable and potentially two
writes and RMW issues i would suggest this:
+ if (buf[0] == '1')
+ s->flags |= SLAB_FAILSLAB;
+ else
+ s->flags &= ~SLAB_FAILSLAB;
This way also has RMW issues, and also the compiler is allowed to
temporarily modify s->flags any way it likes; with WRITE_ONCE() it can't.
Okay, so the safest way is this?
if (buf[0] == '1')
WRITE_ONCE(s->flags, READ_ONCE(s->flags) | SLAB_FAILSLAB);
else
WRITE_ONCE(s->flags, READ_ONCE(s->flags) & ~SLAB_FAILSLAB);
It got me thinking how many places would break if the compiler
starts to temporariliy modify the flags - i hope it never does.
--
Regards,
Alexander Atanasov
