[tip:locking/core] locking/spinlocks: Document the semantics of spin_is_locked()

2018-05-15 Thread tip-bot for Andrea Parri 
Commit-ID:  b7e4aadef28f217de8907eec60a964328797a2be
Gitweb: https://git.kernel.org/tip/b7e4aadef28f217de8907eec60a964328797a2be
Author: Andrea Parri 
AuthorDate: Mon, 14 May 2018 16:01:27 -0700
Committer:  Ingo Molnar 
CommitDate: Tue, 15 May 2018 08:11:15 +0200

locking/spinlocks: Document the semantics of spin_is_locked()

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=151981440005264=2
https://marc.info/?l=linux-kernel=152042843808540=2
https://marc.info/?l=linux-kernel=152043346110262=2

Co-Developed-by: Andrea Parri 
Co-Developed-by: Alan Stern 
Co-Developed-by: David Howells 
Signed-off-by: Andrea Parri 
Signed-off-by: Alan Stern 
Signed-off-by: David Howells 
Signed-off-by: Paul E. McKenney 
Acked-by: Randy Dunlap 
Cc: Akira Yokosawa 
Cc: Andrew Morton 
Cc: Boqun Feng 
Cc: Jade Alglave 
Cc: Linus Torvalds 
Cc: Luc Maranget 
Cc: Nicholas Piggin 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
Cc: Will Deacon 
Cc: linux-a...@vger.kernel.org
Cc: parri.and...@gmail.com
Link: 
http://lkml.kernel.org/r/1526338889-7003-1-git-send-email-paul...@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar 
---
 include/linux/spinlock.h | 18 ++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 4894d322d258..1e8a46435838 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.
+ *
+ * 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(>rlock);

[tip:locking/core] locking/spinlocks: Document the semantics of spin_is_locked()

2018-05-15 Thread tip-bot for Andrea Parri 
Commit-ID:  b7e4aadef28f217de8907eec60a964328797a2be
Gitweb: https://git.kernel.org/tip/b7e4aadef28f217de8907eec60a964328797a2be
Author: Andrea Parri 
AuthorDate: Mon, 14 May 2018 16:01:27 -0700
Committer:  Ingo Molnar 
CommitDate: Tue, 15 May 2018 08:11:15 +0200

locking/spinlocks: Document the semantics of spin_is_locked()

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=151981440005264=2
https://marc.info/?l=linux-kernel=152042843808540=2
https://marc.info/?l=linux-kernel=152043346110262=2

Co-Developed-by: Andrea Parri 
Co-Developed-by: Alan Stern 
Co-Developed-by: David Howells 
Signed-off-by: Andrea Parri 
Signed-off-by: Alan Stern 
Signed-off-by: David Howells 
Signed-off-by: Paul E. McKenney 
Acked-by: Randy Dunlap 
Cc: Akira Yokosawa 
Cc: Andrew Morton 
Cc: Boqun Feng 
Cc: Jade Alglave 
Cc: Linus Torvalds 
Cc: Luc Maranget 
Cc: Nicholas Piggin 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
Cc: Will Deacon 
Cc: linux-a...@vger.kernel.org
Cc: parri.and...@gmail.com
Link: 
http://lkml.kernel.org/r/1526338889-7003-1-git-send-email-paul...@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar 
---
 include/linux/spinlock.h | 18 ++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 4894d322d258..1e8a46435838 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.
+ *
+ * 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(>rlock);