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

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commit 92e75428ffc90e2a0321062379f883f3671cfebe upstream.

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>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Theodore Ts'o 2017-06-07 19:01:32 -04:00 committed by Greg Kroah-Hartman
parent db203ab240
commit 031987b6be
1 changed files with 6 additions and 6 deletions
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12
drivers/char/random.c
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@ -886,15 +886,15 @@ static void add_interrupt_bench(cycles_t start)
static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs) static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
{ {
__u32 *ptr = (__u32 *) regs; __u32 *ptr = (__u32 *) regs;
unsigned long flags; unsigned int idx;
if (regs == NULL) if (regs == NULL)
return 0; return 0;
local_irq_save(flags); idx = READ_ONCE(f->reg_idx);
if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32)) if (idx >= sizeof(struct pt_regs) / sizeof(__u32))
f->reg_idx = 0; idx = 0;
ptr += f->reg_idx++; ptr += idx++;
local_irq_restore(flags); WRITE_ONCE(f->reg_idx, idx);
return *ptr; return *ptr;
} }