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Date: Sun, 12 Apr 2009 13:32:54 +0200 From: Jan Blunck <jblunck@...e.de> To: "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com> Cc: Nick Piggin <npiggin@...e.de>, Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well Am 11.04.2009 um 19:49 schrieb "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com >: > On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck 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. > > The thought of calling _atomic_dec_and_lock() when you already hold > the > lock really really scares me. > > Could you please give an example where you need to do this? > There is a part of the union mount patches that needs to do a union_put() (which itself includes a path_put() that uses atomic_dec_and_lock() in mntput() ). Since it is changing the namespace I need to hold the vfsmount lock. I know that the mnt's count > 1 since it is a parent of the mnt I'm changing in the mount tree. I could possibly delay the union_put(). In general this let's atomic_dec_and_lock() behave similar on SMP and UP. Remember that this already works with CONFIG_SMP as before Nick's patch. > Thanx, Paul > >> Signed-off-by: Jan Blunck <jblunck@...e.de> >> --- >> 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)) >> -- >> 1.6.0.2 >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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