The patch titled
Subject: spinlocks.txt: add a discussion on why spin_is_locked() is bad
has been added to the -mm tree. Its filename is
spinlockstxt-add-a-discussion-on-why-spin_is_locked-is-bad.patch
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------------------------------------------------------
From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Subject: spinlocks.txt: add a discussion on why spin_is_locked() is bad
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Wolfram Sang <w.sang@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/spinlocks.txt | 43 ++++++++++++++++++++++++++++++++++
1 file changed, 43 insertions(+)
diff -puN Documentation/spinlocks.txt~spinlockstxt-add-a-discussion-on-why-spin_is_locked-is-bad Documentation/spinlocks.txt
--- a/Documentation/spinlocks.txt~spinlockstxt-add-a-discussion-on-why-spin_is_locked-is-bad
+++ a/Documentation/spinlocks.txt
@@ -146,6 +146,49 @@ indeed), while write-locks need to prote
----
+spin_is_locked is a bad idea
+
+spin_is_locked checks if a lock is currently hold. On uniprocessor kernels
+it always returns 0. In general this function should be avoided because most
+uses of it are either redundant or broken.
+
+People often use spin_is_locked() to check if a particular lock is hold when a function
+is called to enforce a locking discipline, like
+
+ WARN_ON(!spin_is_locked(!my_lock))
+
+or
+
+ BUG_ON(!spin_is_locked(!my_lock))
+
+or some variant of those.
+
+This does not work on uniprocessor kernels because they will always fail.
+While there are ways around that they are ugly and not recommended.
+Better use lockdep_assert_held(). This also only checks on a lock debugging
+kernel (which you should occasionally run on your code anyways because
+it catches many more problems).
+
+In generally this would be better done with static annotation anyways
+(there's some support for it in sparse)
+
+ BUG_ON(spin_is_locked(obj->lock));
+ kfree(obj);
+
+Another usage is checking whether a lock is not hold when freeing an object.
+However this is redundant because lock debugging supports this anyways
+without explicit code. Just delete the BUG_ON.
+
+A third usage is to check in a console function if a lock is hold, to get
+a panic crash dump out even when some other thread died in it.
+This is better implemented with spin_try_lock() et.al. and a timeout.
+
+Other usages are usually simply races.
+
+In summary just don't use it.
+
+----
+
Reference information:
For dynamic initialization, use spin_lock_init() or rwlock_init() as
_
Subject: Subject: spinlocks.txt: add a discussion on why spin_is_locked() is bad
Patches currently in -mm which might be from ak@xxxxxxxxxxxxxxx are
origin.patch
brlocks-lglocks-cleanups.patch
block-use-lockdep_assert_held-for-queue-locking.patch
sgi-xp-use-lockdep_assert_held.patch
drivers-scsi-aha152xc-remove-broken-usage-of-spin_is_locked.patch
xfs-fix-lock-assert-on-up.patch
mm-huge_memoryc-use-lockdep_assert_held.patch
futex-use-lockdep_assert_held-for-lock-checking.patch
drivers-net-irda-sir_devc-remove-spin_is_locked.patch
drivers-net-ethernet-smsc-smsc911xh-use-lockdep_assert_held-instead-of-home-grown-buggy-construct.patch
spinlockstxt-add-a-discussion-on-why-spin_is_locked-is-bad.patch
include-linux-spinlockh-add-a-kerneldoc-comment-to-spin_is_locked-that-discourages-its-use.patch
checkpatch-check-for-spin_is_locked.patch
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