once: add DO_ONCE_SLOW() for sleepable contexts

[ Upstream commit 62c07983bef9d3e78e71189441e1a470f0d1e653 ]

Christophe Leroy reported a ~80ms latency spike
happening at first TCP connect() time.

This is because __inet_hash_connect() uses get_random_once()
to populate a perturbation table which became quite big
after commit 4c2c8f03a5ab ("tcp: increase source port perturb table to 2^16")

get_random_once() uses DO_ONCE(), which block hard irqs for the duration
of the operation.

This patch adds DO_ONCE_SLOW() which uses a mutex instead of a spinlock
for operations where we prefer to stay in process context.

Then __inet_hash_connect() can use get_random_slow_once()
to populate its perturbation table.

Fixes: 4c2c8f03a5ab ("tcp: increase source port perturb table to 2^16")
Fixes: 190cc82489f4 ("tcp: change source port randomizarion at connect() time")
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/netdev/CANn89iLAEYBaoYajy0Y9UmGFff5GPxDUoG-ErVB2jDdRNQ5Tug@mail.gmail.com/T/#t
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willy Tarreau <w@1wt.eu>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/linux/once.h

@@ -5,10 +5,18 @@
 #include <linux/types.h>
 #include <linux/jump_label.h>
 
+/* Helpers used from arbitrary contexts.
+ * Hard irqs are blocked, be cautious.
+ */
 bool __do_once_start(bool *done, unsigned long *flags);
 void __do_once_done(bool *done, struct static_key_true *once_key,
 		    unsigned long *flags, struct module *mod);
 
+/* Variant for process contexts only. */
+bool __do_once_slow_start(bool *done);
+void __do_once_slow_done(bool *done, struct static_key_true *once_key,
+			 struct module *mod);
+
 /* Call a function exactly once. The idea of DO_ONCE() is to perform
  * a function call such as initialization of random seeds, etc, only
  * once, where DO_ONCE() can live in the fast-path. After @func has
@@ -52,9 +60,29 @@ void __do_once_done(bool *done, struct static_key_true *once_key,
 		___ret;							     \
 	})
 
+/* Variant of DO_ONCE() for process/sleepable contexts. */
+#define DO_ONCE_SLOW(func, ...)						     \
+	({								     \
+		bool ___ret = false;					     \
+		static bool __section(".data.once") ___done = false;	     \
+		static DEFINE_STATIC_KEY_TRUE(___once_key);		     \
+		if (static_branch_unlikely(&___once_key)) {		     \
+			___ret = __do_once_slow_start(&___done);	     \
+			if (unlikely(___ret)) {				     \
+				func(__VA_ARGS__);			     \
+				__do_once_slow_done(&___done, &___once_key,  \
+						    THIS_MODULE);	     \
+			}						     \
+		}							     \
+		___ret;							     \
+	})
+
 #define get_random_once(buf, nbytes)					     \
 	DO_ONCE(get_random_bytes, (buf), (nbytes))
 #define get_random_once_wait(buf, nbytes)                                    \
 	DO_ONCE(get_random_bytes_wait, (buf), (nbytes))                      \
 
+#define get_random_slow_once(buf, nbytes)				     \
+	DO_ONCE_SLOW(get_random_bytes, (buf), (nbytes))
+
 #endif /* _LINUX_ONCE_H */