random: use lockless method of accessing and updating f->reg_idx

Linus pointed out that there is a much more efficient way of avoiding
the problem that we were trying to address in commit 9dfa7bba35ac0:
"fix race in drivers/char/random.c:get_reg()".

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a561f0c2f428df6cbd80e0fe5bfd3c479a578e18..473ad34378f2cb25b8e3a941ed388b9904648e2a 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1097,15 +1097,15 @@ static void add_interrupt_bench(cycles_t start)
 static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 {
        __u32 *ptr = (__u32 *) regs;
-       unsigned long flags;
+       unsigned int idx;
 
        if (regs == NULL)
                return 0;
-       local_irq_save(flags);
-       if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))
-               f->reg_idx = 0;
-       ptr += f->reg_idx++;
-       local_irq_restore(flags);
+       idx = READ_ONCE(f->reg_idx);
+       if (idx >= sizeof(struct pt_regs) / sizeof(__u32))
+               idx = 0;
+       ptr += idx++;
+       WRITE_ONCE(f->reg_idx, idx);
        return *ptr;
 }