On Wed, 2022-10-12 at 21:29 +0000, David Laight wrote:

From: Joe Perches

quoted

Sent: 12 October 2022 20:17

On Wed, 2022-10-05 at 23:48 +0200, Jason A. Donenfeld wrote:

quoted

The prandom_u32() function has been a deprecated inline wrapper around
get_random_u32() for several releases now, and compiles down to the
exact same code. Replace the deprecated wrapper with a direct call to
the real function.

[]

quoted

diff --git a/drivers/infiniband/hw/cxgb4/cm.c b/drivers/infiniband/hw/cxgb4/cm.c

[]

quoted

@@ -734,7 +734,7 @@ static int send_connect(struct c4iw_ep *ep)
 				   &ep->com.remote_addr;
 	int ret;
 	enum chip_type adapter_type = ep->com.dev->rdev.lldi.adapter_type;
-	u32 isn = (prandom_u32() & ~7UL) - 1;
+	u32 isn = (get_random_u32() & ~7UL) - 1;

trivia:

There are somewhat odd size mismatches here.

I had to think a tiny bit if random() returned a value from 0 to 7
and was promoted to a 64 bit value then truncated to 32 bit.

Perhaps these would be clearer as ~7U and not ~7UL

That makes no difference - the compiler will generate the same code.

True, more or less.  It's more a question for the reader.

The real question is WTF is the code doing?

True.

The '& ~7u' clears the bottom 3 bits.
The '- 1' then sets the bottom 3 bits and decrements the
(random) high bits.

Right.

So is the same as get_random_u32() | 7.

True, it's effectively the same as the upper 29 bits are random
anyway and the bottom 3 bits are always set.

But I bet the coder had something else in mind.

Likely.

And it was also likely copy/pasted a few times.