From b4f19537f9d26dbf95bd2e7d9c056400ad1b5723 Mon Sep 17 00:00:00 2001 From: Tulio Magno Quites Machado Filho Date: Wed, 22 Jul 2015 09:26:02 -0300 Subject: PowerPC: Fix a race condition when eliding a lock 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) (cherry picked from commit 5b319ce2949cf6fb97862ff81558944f76c704f1) --- sysdeps/powerpc/nptl/elide.h | 115 ++++++++++++++++++++++++------------------- 1 file changed, 63 insertions(+), 52 deletions(-) diff --git a/sysdeps/powerpc/nptl/elide.h b/sysdeps/powerpc/nptl/elide.h index 389f5a5e9e..12171f45dc 100644 --- a/sysdeps/powerpc/nptl/elide.h +++ b/sysdeps/powerpc/nptl/elide.h @@ -23,67 +23,78 @@ # include # include -/* 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 -- cgit v1.2.1