summaryrefslogtreecommitdiff
path: root/include
diff options
context:
space:
mode:
authorManfred Spraul <manfred@colorfullife.com>2016-10-11 13:54:50 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-10-28 03:01:32 -0400
commitf6031d95320dc2930f1b813e98533a01c92f3dc0 (patch)
tree68741eceb18f0e17330775b423851b2f0239e217 /include
parentb52b7b5a5cafe1b415291b971259cee319345d18 (diff)
ipc/sem.c: fix complex_count vs. simple op race
commit 5864a2fd3088db73d47942370d0f7210a807b9bc upstream. Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a race: sem_lock has a fast path that allows parallel simple operations. There are two reasons why a simple operation cannot run in parallel: - a non-simple operations is ongoing (sma->sem_perm.lock held) - a complex operation is sleeping (sma->complex_count != 0) As both facts are stored independently, a thread can bypass the current checks by sleeping in the right positions. See below for more details (or kernel bugzilla 105651). The patch fixes that by creating one variable (complex_mode) that tracks both reasons why parallel operations are not possible. The patch also updates stale documentation regarding the locking. With regards to stable kernels: The patch is required for all kernels that include the commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") (3.10?) The alternative is to revert the patch that introduced the race. The patch is safe for backporting, i.e. it makes no assumptions about memory barriers in spin_unlock_wait(). Background: Here is the race of the current implementation: Thread A: (simple op) - does the first "sma->complex_count == 0" test Thread B: (complex op) - does sem_lock(): This includes an array scan. But the scan can't find Thread A, because Thread A does not own sem->lock yet. - the thread does the operation, increases complex_count, drops sem_lock, sleeps Thread A: - spin_lock(&sem->lock), spin_is_locked(sma->sem_perm.lock) - sleeps before the complex_count test Thread C: (complex op) - does sem_lock (no array scan, complex_count==1) - wakes up Thread B. - decrements complex_count Thread A: - does the complex_count test Bug: Now both thread A and thread C operate on the same array, without any synchronization. Fixes: 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") Link: http://lkml.kernel.org/r/1469123695-5661-1-git-send-email-manfred@colorfullife.com Reported-by: <felixh@informatik.uni-bremen.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Cc: <1vier1@web.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/sem.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/include/linux/sem.h b/include/linux/sem.h
index 976ce3a19f1b..d0efd6e6c20a 100644
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -21,6 +21,7 @@ struct sem_array {
struct list_head list_id; /* undo requests on this array */
int sem_nsems; /* no. of semaphores in array */
int complex_count; /* pending complex operations */
+ bool complex_mode; /* no parallel simple ops */
};
#ifdef CONFIG_SYSVIPC