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random: don't try to look at entropy_count outside the lock

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As a non-atomic value, it's only safe to look at entropy_count when the
pool lock is held, so we move the BUG_ON inside the lock for correctness.

Also remove the spurious comment.  It's ok for entropy_count to
temporarily exceed POOLBITS so long as it's left in a consistent state
when the lock is released.

This is a more correct, simple, and idiomatic fix for the bug in
8b76f46a2d.  I've left the reorderings introduced by that patch in place
as they're harmless, even though they don't properly deal with potential
atomicity issues.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matt Mackall 2009-01-06 14:42:55 -08:00 committed by Linus Torvalds
parent 71183c9480
commit cda796a3d5
1 changed files with 2 additions and 3 deletions
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5
drivers/char/random.c
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@ -407,7 +407,7 @@ struct entropy_store {
/* read-write data: */
spinlock_t lock;
unsigned add_ptr;
int entropy_count; /* Must at no time exceed ->POOLBITS! */
int entropy_count;
int input_rotate;
};
@ -767,11 +767,10 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
{
unsigned long flags;
BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);
/* Hold lock while accounting */
spin_lock_irqsave(&r->lock, flags);
BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);
DEBUG_ENT("trying to extract %d bits from %s\n",
nbytes * 8, r->name);