Messages in this thread | | | Date | Sun, 8 Apr 2018 14:14:46 -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 11:58:25PM +0200, Andrea Parri wrote: > On Fri, Apr 06, 2018 at 02:08:16PM -0700, Randy Dunlap wrote: > > On 04/06/2018 02:07 PM, Paul E. McKenney wrote: > > > 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? > > > > Please change this patch to current doc syntax. > > Any changes to kernel-doc syntax would come later.
Are you sure?
$ git grep "\* Returns:" | wc -l 2470 $ git grep "\* Return:" | wc -l 4144
Looks like more than a third of them are already "Returns:". ;-)
> Paul: I understand that you're going to do this change "in place"; please > let me know if I'm wrong/if you need a new submission.
If Randy is certain that he would like to continue propagating this grammatical infelicity, sure. ;-)
Thanx, Paul
> Thanks, > Andrea > > > > > > Thanks. > > > > > 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 > > >> > > > > > > > > > -- > > ~Randy >
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