Messages in this thread |  | | Subject | Re: [PATCH v4 1/3] locking: Document the semantics of spin_is_locked() | From | Randy Dunlap <> | Date | Fri, 6 Apr 2018 14:01:41 -0700 |
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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.]
> + * > + * 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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