Messages in this thread |  | | Date | Fri, 6 Apr 2018 14:07:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v4 1/3] locking: Document the semantics of spin_is_locked() |
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On Fri, Apr 06, 2018 at 02:01:41PM -0700, Randy Dunlap wrote: > On 04/06/2018 12:47 PM, Andrea Parri wrote: > > There appeared to be a certain, recurrent uncertainty concerning the > > semantics of spin_is_locked(), likely a consequence of the fact that > > this semantics remains undocumented or that it has been historically > > linked to the (likewise unclear) semantics of spin_unlock_wait(). > > > > A recent auditing [1] of the callers of the primitive confirmed that > > none of them are relying on particular ordering guarantees; document > > this semantics by adding a docbook header to spin_is_locked(). Also, > > describe behaviors specific to certain CONFIG_SMP=n builds. > > > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > > https://marc.info/?l=linux-kernel&m=152042843808540&w=2 > > https://marc.info/?l=linux-kernel&m=152043346110262&w=2 > > > > Co-Developed-by: Andrea Parri <andrea.parri@amarulasolutions.com> > > Co-Developed-by: Alan Stern <stern@rowland.harvard.edu> > > Co-Developed-by: David Howells <dhowells@redhat.com> > > Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com> > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu> > > Signed-off-by: David Howells <dhowells@redhat.com> > > Cc: Will Deacon <will.deacon@arm.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Boqun Feng <boqun.feng@gmail.com> > > Cc: Nicholas Piggin <npiggin@gmail.com> > > Cc: Jade Alglave <j.alglave@ucl.ac.uk> > > Cc: Luc Maranget <luc.maranget@inria.fr> > > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > Cc: Akira Yokosawa <akiyks@gmail.com> > > Cc: Ingo Molnar <mingo@kernel.org> > > --- > > include/linux/spinlock.h | 18 ++++++++++++++++++ > > 1 file changed, 18 insertions(+) > > > > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h > > index 4894d322d2584..1e8a464358384 100644 > > --- a/include/linux/spinlock.h > > +++ b/include/linux/spinlock.h > > @@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock) > > raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ > > }) > > > > +/** > > + * spin_is_locked() - Check whether a spinlock is locked. > > + * @lock: Pointer to the spinlock. > > + * > > + * This function is NOT required to provide any memory ordering > > + * guarantees; it could be used for debugging purposes or, when > > + * additional synchronization is needed, accompanied with other > > + * constructs (memory barriers) enforcing the synchronization. > > + * > > + * Returns: 1 if @lock is locked, 0 otherwise. > > Sorry, minor nit: > s/Returns:/Return:/ > (according to kernel-doc.rst) > > although I agree that "Returns:" is better. > [I should have changed that years ago.]
Agreed, English grammar and templates often seem to conflict.
So should we change this comment, or are you instead proposing to add "Returns:" as valid kernel-doc?
Thanx, Paul
> > + * > > + * Note that the function only tells you that the spinlock is > > + * seen to be locked, not that it is locked on your CPU. > > + * > > + * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n, > > + * the return value is always 0 (see include/linux/spinlock_up.h). > > + * Therefore you should not rely heavily on the return value. > > + */ > > static __always_inline int spin_is_locked(spinlock_t *lock) > > { > > return raw_spin_is_locked(&lock->rlock); > > > > > -- > ~Randy >
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