Messages in this thread | | | Date | Fri, 17 Apr 2009 15:14:05 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well |
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On Fri, 10 Apr 2009 18:13:57 +0200 Jan Blunck <jblunck@suse.de> wrote:
> I think it is wrong to unconditionally take the lock before calling > atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in > situation where it is known that the counter will not reach zero (e.g. holding > another reference to the same object) but the lock is already taken. >
It can't deadlock, because spin_lock() doesn't do anything on CONFIG_SMP=n.
You might get lockdep whines on CONFIG_SMP=n, but they'd be false positives because lockdep doesn't know that we generate additional code for SMP builds.
> --- > lib/dec_and_lock.c | 3 +-- > 1 files changed, 1 insertions(+), 2 deletions(-) > > diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c > index a65c314..e73822a 100644 > --- a/lib/dec_and_lock.c > +++ b/lib/dec_and_lock.c > @@ -19,11 +19,10 @@ > */ > int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock) > { > -#ifdef CONFIG_SMP > /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */ > if (atomic_add_unless(atomic, -1, 1)) > return 0; > -#endif > + > /* Otherwise do it the slow way */ > spin_lock(lock); > if (atomic_dec_and_test(atomic))
The patch looks reasonable from a cleanup/consistency POV, but the analysis and changelog need a bit of help, methinks.
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