linux-kernel - [PATCH v4 1/2] lib/vsprintf: Remove static_branch_likely() from __ptr_to

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Date:   Tue, 27 Sep 2022 12:49:11 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Theodore Ts'o <tytso@....edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Jason A . Donenfeld " <Jason@...c4.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v4 1/2] lib/vsprintf: Remove static_branch_likely() from __ptr_to_hashval().

Using static_branch_likely() to signal that ptr_key has been filled is a
bit much given that it is not a fast path.

Replace static_branch_likely() with bool for condition and a memory
barrier for ptr_key.

Suggested-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 lib/vsprintf.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3c1853a9d1c09..bce63cbf23779 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -750,12 +750,7 @@ static int __init debug_boot_weak_hash_enable(char *str)
 }
 early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable);
 
-static DEFINE_STATIC_KEY_FALSE(filled_random_ptr_key);
-
-static void enable_ptr_key_workfn(struct work_struct *work)
-{
-	static_branch_enable(&filled_random_ptr_key);
-}
+static bool filled_random_ptr_key __read_mostly;
 
 /* Maps a pointer to a 32 bit unique identifier. */
 static inline int __ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
@@ -763,24 +758,26 @@ static inline int __ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
 	static siphash_key_t ptr_key __read_mostly;
 	unsigned long hashval;
 
-	if (!static_branch_likely(&filled_random_ptr_key)) {
+	if (!READ_ONCE(filled_random_ptr_key)) {
 		static bool filled = false;
 		static DEFINE_SPINLOCK(filling);
-		static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);
 		unsigned long flags;
 
-		if (!system_unbound_wq || !rng_is_initialized() ||
+		if (!rng_is_initialized() ||
 		    !spin_trylock_irqsave(&filling, flags))
 			return -EAGAIN;
 
 		if (!filled) {
 			get_random_bytes(&ptr_key, sizeof(ptr_key));
-			queue_work(system_unbound_wq, &enable_ptr_key_work);
+			/* Pairs with smp_rmb() before reading ptr_key. */
+			smp_wmb();
+			WRITE_ONCE(filled_random_ptr_key, true);
 			filled = true;
 		}
 		spin_unlock_irqrestore(&filling, flags);
 	}
-
+	/* Pairs with smp_wmb() after writing ptr_key. */
+	smp_rmb();
 
 #ifdef CONFIG_64BIT
 	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
-- 
2.37.2