[COMMITTED,2.22] PowerPC: Fix a race condition when eliding a lock
Commit Message
The previous code used to evaluate the preprocessor token is_lock_free to
a variable before starting a transaction. This behavior can cause an
error if another thread got the lock (without using a transaction)
between the evaluation of the token and the beginning of the transaction.
This bug can be triggered with the following order of events:
1. The lock accessed by is_lock_free is free.
2. Thread T1 evaluates is_lock_free and stores into register R1 that the
lock is free.
3. Thread T2 acquires the same lock used in is_lock_free.
4. T1 begins the transaction, creating a memory barrier where is_lock_free
is false, but R1 is true.
5. T1 reads R1 and doesn't abort the transaction.
6. T1 calls ELIDE_UNLOCK, which reads false from is_lock_free and decides
to unlock a lock acquired by T2, leading to undefined behavior.
This patch delays the evaluation of is_lock_free to inside a transaction
by moving this part of the code to the macro ELIDE_LOCK.
[BZ #18743]
* sysdeps/powerpc/nptl/elide.h (__elide_lock): Move most of this
code to...
(ELIDE_LOCK): ...here.
(__get_new_count): New function with part of the code from
__elide_lock that updates the value of adapt_count after a
transaction abort.
(__elided_trylock): Moved this code to...
(ELIDE_TRYLOCK): ...here.
(cherry picked from commit 6ec52bf634b7650b57ff67b5f5053bce8992d549)
---
ChangeLog | 12 +++++
NEWS | 4 +-
sysdeps/powerpc/nptl/elide.h | 115 ++++++++++++++++++++++++-------------------
3 files changed, 77 insertions(+), 54 deletions(-)
@@ -1,3 +1,15 @@
+2015-10-20 Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
+
+ [BZ #18743]
+ * sysdeps/powerpc/nptl/elide.h (__elide_lock): Move most of this
+ code to...
+ (ELIDE_LOCK): ...here.
+ (__get_new_count): New function with part of the code from
+ __elide_lock that updates the value of adapt_count after a
+ transaction abort.
+ (__elided_trylock): Moved this code to...
+ (ELIDE_TRYLOCK): ...here.
+
2015-10-06 Florian Weimer <fweimer@redhat.com>
[BZ #19018]
@@ -9,8 +9,8 @@ Version 2.22.1
* The following bugs are resolved with this release:
- 18589, 18778, 18781, 18787, 18796, 18870, 18887, 18921, 18928, 18969,
- 19018.
+ 18589, 18743, 18778, 18781, 18787, 18796, 18870, 18887, 18921, 18928,
+ 18969, 19018.
* The LD_POINTER_GUARD environment variable can no longer be used to
disable the pointer guard feature. It is always enabled.
@@ -23,67 +23,78 @@
# include <htm.h>
# include <elision-conf.h>
-/* Returns true if the lock defined by is_lock_free as elided.
- ADAPT_COUNT is a pointer to per-lock state variable. */
-
+/* Get the new value of adapt_count according to the elision
+ configurations. Returns true if the system should retry again or false
+ otherwise. */
static inline bool
-__elide_lock (uint8_t *adapt_count, int is_lock_free)
+__get_new_count (uint8_t *adapt_count)
{
- if (*adapt_count > 0)
+ /* A persistent failure indicates that a retry will probably
+ result in another failure. Use normal locking now and
+ for the next couple of calls. */
+ if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
{
- (*adapt_count)--;
+ if (__elision_aconf.skip_lock_internal_abort > 0)
+ *adapt_count = __elision_aconf.skip_lock_internal_abort;
return false;
}
-
- for (int i = __elision_aconf.try_tbegin; i > 0; i--)
- {
- if (__builtin_tbegin (0))
- {
- if (is_lock_free)
- return true;
- /* Lock was busy. */
- __builtin_tabort (_ABORT_LOCK_BUSY);
- }
- else
- {
- /* A persistent failure indicates that a retry will probably
- result in another failure. Use normal locking now and
- for the next couple of calls. */
- if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
- {
- if (__elision_aconf.skip_lock_internal_abort > 0)
- *adapt_count = __elision_aconf.skip_lock_internal_abort;
- break;
- }
- /* Same logic as above, but for a number of temporary failures in a
- a row. */
- else if (__elision_aconf.skip_lock_out_of_tbegin_retries > 0
- && __elision_aconf.try_tbegin > 0)
- *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
- }
- }
-
- return false;
+ /* Same logic as above, but for a number of temporary failures in a
+ a row. */
+ else if (__elision_aconf.skip_lock_out_of_tbegin_retries > 0
+ && __elision_aconf.try_tbegin > 0)
+ *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
+ return true;
}
-# define ELIDE_LOCK(adapt_count, is_lock_free) \
- __elide_lock (&(adapt_count), is_lock_free)
-
-
-static inline bool
-__elide_trylock (uint8_t *adapt_count, int is_lock_free, int write)
-{
- if (__elision_aconf.try_tbegin > 0)
- {
- if (write)
- __builtin_tabort (_ABORT_NESTED_TRYLOCK);
- return __elide_lock (adapt_count, is_lock_free);
- }
- return false;
-}
+/* CONCURRENCY NOTES:
+
+ The evaluation of the macro expression is_lock_free encompasses one or
+ more loads from memory locations that are concurrently modified by other
+ threads. For lock elision to work, this evaluation and the rest of the
+ critical section protected by the lock must be atomic because an
+ execution with lock elision must be equivalent to an execution in which
+ the lock would have been actually acquired and released. Therefore, we
+ evaluate is_lock_free inside of the transaction that represents the
+ critical section for which we want to use lock elision, which ensures
+ the atomicity that we require. */
+
+/* Returns 0 if the lock defined by is_lock_free was elided.
+ ADAPT_COUNT is a per-lock state variable. */
+# define ELIDE_LOCK(adapt_count, is_lock_free) \
+ ({ \
+ int ret = 0; \
+ if (adapt_count > 0) \
+ (adapt_count)--; \
+ else \
+ for (int i = __elision_aconf.try_tbegin; i > 0; i--) \
+ { \
+ if (__builtin_tbegin (0)) \
+ { \
+ if (is_lock_free) \
+ { \
+ ret = 1; \
+ break; \
+ } \
+ __builtin_tabort (_ABORT_LOCK_BUSY); \
+ } \
+ else \
+ if (!__get_new_count(&adapt_count)) \
+ break; \
+ } \
+ ret; \
+ })
# define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) \
- __elide_trylock (&(adapt_count), is_lock_free, write)
+ ({ \
+ int ret = 0; \
+ if (__elision_aconf.try_tbegin > 0) \
+ { \
+ if (write) \
+ __builtin_tabort (_ABORT_NESTED_TRYLOCK); \
+ ret = ELIDE_LOCK (adapt_count, is_lock_free); \
+ } \
+ ret; \
+ })
static inline bool