summaryrefslogtreecommitdiff
path: root/include/linux/rwsem.h
diff options
context:
space:
mode:
authorWaiman Long <longman@redhat.com>2018-09-06 23:18:34 +0300
committerIngo Molnar <mingo@kernel.org>2018-09-10 13:04:07 +0300
commit925b9cd1b89a94b7124d128c80dfc48f78a63098 (patch)
treee15169c03e608800695a006b6b812fa9df0a35d4 /include/linux/rwsem.h
parent4b486b535c33ef354ecf02a2650919004fd7d2b0 (diff)
downloadlinux-925b9cd1b89a94b7124d128c80dfc48f78a63098.tar.xz
locking/rwsem: Make owner store task pointer of last owning reader
Currently, when a reader acquires a lock, it only sets the RWSEM_READER_OWNED bit in the owner field. The other bits are simply not used. When debugging hanging cases involving rwsems and readers, the owner value does not provide much useful information at all. This patch modifies the current behavior to always store the task_struct pointer of the last rwsem-acquiring reader in a reader-owned rwsem. This may be useful in debugging rwsem hanging cases especially if only one reader is involved. However, the task in the owner field may not the real owner or one of the real owners at all when the owner value is examined, for example, in a crash dump. So it is just an additional hint about the past history. If CONFIG_DEBUG_RWSEMS=y is enabled, the owner field will be checked at unlock time too to make sure the task pointer value is valid. That does have a slight performance cost and so is only enabled as part of that debug option. From the performance point of view, it is expected that the changes shouldn't have any noticeable performance impact. A rwsem microbenchmark (with 48 worker threads and 1:1 reader/writer ratio) was ran on a 2-socket 24-core 48-thread Haswell system. The locking rates on a 4.19-rc1 based kernel were as follows: 1) Unpatched kernel: 543.3 kops/s 2) Patched kernel: 549.2 kops/s 3) Patched kernel (CONFIG_DEBUG_RWSEMS on): 546.6 kops/s There was actually a slight increase in performance (1.1%) in this particular case. Maybe it was caused by the elimination of a branch or just a testing noise. Turning on the CONFIG_DEBUG_RWSEMS option also had less than the expected impact on performance. The least significant 2 bits of the owner value are now used to designate the rwsem is readers owned and the owners are anonymous. Signed-off-by: Waiman Long <longman@redhat.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Link: http://lkml.kernel.org/r/1536265114-10842-1-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'include/linux/rwsem.h')
-rw-r--r--include/linux/rwsem.h4
1 files changed, 2 insertions, 2 deletions
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index ab93b6eae696..67dbb57508b1 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -45,10 +45,10 @@ struct rw_semaphore {
};
/*
- * Setting bit 0 of the owner field with other non-zero bits will indicate
+ * Setting bit 1 of the owner field but not bit 0 will indicate
* that the rwsem is writer-owned with an unknown owner.
*/
-#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-1L)
+#define RWSEM_OWNER_UNKNOWN ((struct task_struct *)-2L)
extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);