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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>
This commit is contained in:
Theodore Ts'o 2017-06-07 19:01:32 -04:00
parent 9dfa7bba35
commit 92e75428ff
1 changed files with 6 additions and 6 deletions
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12
drivers/char/random.c
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@ -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;
}