Re: [PATCH v2 4/5] util/qht: use striped locks under TSAN

[Alex Bennée]

Emilio Cota <cota@braap.org> writes:

> Fixes this tsan crash, easy to reproduce with any large enough program:
>
> $ tests/unit/test-qht
> 1..2
> ThreadSanitizer: CHECK failed: sanitizer_deadlock_detector.h:67 
> "((n_all_locks_)) < 
> (((sizeof(all_locks_with_contexts_)/sizeof((all_locks_with_contexts_)[0]))))" 
> (0x40, 0x40) (tid=1821568)
>     #0 __tsan::CheckUnwind() 
> ../../../../src/libsanitizer/tsan/tsan_rtl.cpp:353 (libtsan.so.2+0x90034)
>     #1 __sanitizer::CheckFailed(char const*, int, char const*, unsigned long 
> long, unsigned long long) 
> ../../../../src/libsanitizer/sanitizer_common/sanitizer_termination.cpp:86 
> (libtsan.so.2+0xca555)
>     #2 __sanitizer::DeadlockDetectorTLS<__sanitizer::TwoLevelBitVector<1ul, 
> __sanitizer::BasicBitVector<unsigned long> > >::addLock(unsigned long, 
> unsigned long, unsigned int) 
> ../../../../src/libsanitizer/sanitizer_common/sanitizer_deadlock_detector.h:67
>  (libtsan.so.2+0xb3616)
>     #3 __sanitizer::DeadlockDetectorTLS<__sanitizer::TwoLevelBitVector<1ul, 
> __sanitizer::BasicBitVector<unsigned long> > >::addLock(unsigned long, 
> unsigned long, unsigned int) 
> ../../../../src/libsanitizer/sanitizer_common/sanitizer_deadlock_detector.h:59
>  (libtsan.so.2+0xb3616)
>     #4 __sanitizer::DeadlockDetector<__sanitizer::TwoLevelBitVector<1ul, 
> __sanitizer::BasicBitVector<unsigned long> > 
> >::onLockAfter(__sanitizer::DeadlockDetectorTLS<__sanitizer::TwoLevelBitVector<1ul,
>  __sanitizer::BasicBitVector<unsigned long> > >*, unsigned long, unsigned 
> int) 
> ../../../../src/libsanitizer/sanitizer_common/sanitizer_deadlock_detector.h:216
>  (libtsan.so.2+0xb3616)
>     #5 __sanitizer::DD::MutexAfterLock(__sanitizer::DDCallback*, 
> __sanitizer::DDMutex*, bool, bool) 
> ../../../../src/libsanitizer/sanitizer_common/sanitizer_deadlock_detector1.cpp:169
>  (libtsan.so.2+0xb3616)
>     #6 __tsan::MutexPostLock(__tsan::ThreadState*, unsigned long, unsigned 
> long, unsigned int, int) 
> ../../../../src/libsanitizer/tsan/tsan_rtl_mutex.cpp:200 
> (libtsan.so.2+0xa3382)
>     #7 __tsan_mutex_post_lock 
> ../../../../src/libsanitizer/tsan/tsan_interface_ann.cpp:384 
> (libtsan.so.2+0x76bc3)
>     #8 qemu_spin_lock /home/cota/src/qemu/include/qemu/thread.h:259 
> (test-qht+0x44a97)
>     #9 qht_map_lock_buckets ../util/qht.c:253 (test-qht+0x44a97)
>     #10 do_qht_iter ../util/qht.c:809 (test-qht+0x45f33)
>     #11 qht_iter ../util/qht.c:821 (test-qht+0x45f33)
>     #12 iter_check ../tests/unit/test-qht.c:121 (test-qht+0xe473)
>     #13 qht_do_test ../tests/unit/test-qht.c:202 (test-qht+0xe473)
>     #14 qht_test ../tests/unit/test-qht.c:240 (test-qht+0xe7c1)
>     #15 test_default ../tests/unit/test-qht.c:246 (test-qht+0xe828)
>     #16 <null> <null> (libglib-2.0.so.0+0x7daed)
>     #17 <null> <null> (libglib-2.0.so.0+0x7d80a)
>     #18 <null> <null> (libglib-2.0.so.0+0x7d80a)
>     #19 g_test_run_suite <null> (libglib-2.0.so.0+0x7dfe9)
>     #20 g_test_run <null> (libglib-2.0.so.0+0x7e055)
>     #21 main ../tests/unit/test-qht.c:259 (test-qht+0xd2c6)
>     #22 __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58 
> (libc.so.6+0x29d8f)
>     #23 __libc_start_main_impl ../csu/libc-start.c:392 (libc.so.6+0x29e3f)
>     #24 _start <null> (test-qht+0xdb44)
>
> Signed-off-by: Emilio Cota <cota@braap.org>
> ---
>  util/qht.c | 101 +++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 87 insertions(+), 14 deletions(-)
>
> diff --git a/util/qht.c b/util/qht.c
> index 15866299e6..70cc733d5d 100644
> --- a/util/qht.c
> +++ b/util/qht.c
> @@ -151,6 +151,22 @@ struct qht_bucket {
>  
>  QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > QHT_BUCKET_ALIGN);
>  
> +/*
> + * Under TSAN, we use striped locks instead of one lock per bucket chain.
> + * This avoids crashing under TSAN, since TSAN aborts the program if more 
> than
> + * 64 locks are held (this is a hardcoded limit in TSAN).
> + * When resizing a QHT we grab all the buckets' locks, which can easily
> + * go over TSAN's limit. By using striped locks, we avoid this problem.
> + *
> + * Note: this number must be a power of two for easy index computation.
> + */
> +#define QHT_TSAN_BUCKET_LOCKS_BITS 4
> +#define QHT_TSAN_BUCKET_LOCKS (1 << QHT_TSAN_BUCKET_LOCKS_BITS)
> +
> +struct qht_tsan_lock {
> +    QemuSpin lock;
> +} QEMU_ALIGNED(QHT_BUCKET_ALIGN);
> +
>  /**
>   * struct qht_map - structure to track an array of buckets
>   * @rcu: used by RCU. Keep it as the top field in the struct to help valgrind
> @@ -160,6 +176,7 @@ QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > 
> QHT_BUCKET_ALIGN);
>   * @n_added_buckets: number of added (i.e. "non-head") buckets
>   * @n_added_buckets_threshold: threshold to trigger an upward resize once the
>   *                             number of added buckets surpasses it.
> + * @tsan_bucket_locks: Array of striped locks to be used only under TSAN.
>   *
>   * Buckets are tracked in what we call a "map", i.e. this structure.
>   */
> @@ -169,6 +186,9 @@ struct qht_map {
>      size_t n_buckets;
>      size_t n_added_buckets;
>      size_t n_added_buckets_threshold;
> +#ifdef CONFIG_TSAN
> +    struct qht_tsan_lock tsan_bucket_locks[QHT_TSAN_BUCKET_LOCKS];
> +#endif
>  };
>  
>  /* trigger a resize when n_added_buckets > n_buckets / div */
> @@ -229,10 +249,62 @@ static inline size_t qht_elems_to_buckets(size_t 
> n_elems)
>      return pow2ceil(n_elems / QHT_BUCKET_ENTRIES);
>  }
>  
> -static inline void qht_head_init(struct qht_bucket *b)
> +/*
> + * When using striped locks (i.e. under TSAN), we have to be careful not
> + * to operate on the same lock twice (e.g. when iterating through all 
> buckets).
> + * We achieve this by operating only on each stripe's first matching lock.
> + */
> +static inline void qht_do_if_first_in_stripe(const struct qht_map *map,
> +                                             struct qht_bucket *b,
> +                                             void (*func)(QemuSpin *spin))
> +{
> +#ifdef CONFIG_TSAN
> +    unsigned long bucket_idx = b - map->buckets;
> +    bool is_first_in_stripe = (bucket_idx >> QHT_TSAN_BUCKET_LOCKS_BITS) == 
> 0;
> +    if (is_first_in_stripe) {
> +        unsigned long lock_idx = bucket_idx & (QHT_TSAN_BUCKET_LOCKS - 1);
> +        func(&map->tsan_bucket_locks[lock_idx]);

Hmm I ran into an issue with:

     ../util/qht.c:286:10: error: incompatible pointer types passing 'const 
struct qht_tsan_lock *' to parameter of type 'QemuSpin *' (aka 'struct QemuSpin 
*') [-Werror,-Wincompatible-pointer-types]
    func(&map->tsan_bucket_locks[lock_idx]);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But changing the line to:

    func(&map->tsan_bucket_locks[lock_idx].lock);

still fails because:

../../util/qht.c:266:14: error: passing 'const QemuSpin *' (aka 'const struct 
QemuSpin *') to parameter of type 'QemuSpin *' (aka 'struct QemuSpin *') 
discards qualifiers [\
-Werror,-Wincompatible-pointer-types-discards-qualifiers]                       
                                                                                
             
        func(&map->tsan_bucket_locks[lock_idx].lock);                           
                                                                                
             
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~            

> +    }
> +#else
> +    func(&b->lock);
> +#endif
> +}
> +
> +static inline void qht_bucket_lock_destroy(const struct qht_map *map,
> +                                           struct qht_bucket *b)
> +{
> +    qht_do_if_first_in_stripe(map, b, qemu_spin_destroy);
> +}

Who is meant to be calling this?

> +
> +static inline void qht_bucket_lock_do(const struct qht_map *map,
> +                                      struct qht_bucket *b,
> +                                      void (*func)(QemuSpin *lock))
> +{
> +#ifdef CONFIG_TSAN
> +    unsigned long bucket_idx = b - map->buckets;
> +    unsigned long lock_idx = bucket_idx & (QHT_TSAN_BUCKET_LOCKS - 1);
> +    func(&map->tsan_bucket_locks[lock_idx]);
> +#else
> +    func(&b->lock);
> +#endif
> +}
> +
> +static inline void qht_bucket_lock(const struct qht_map *map,
> +                                   struct qht_bucket *b)
> +{
> +    qht_bucket_lock_do(map, b, qemu_spin_lock);
> +}
> +
> +static inline void qht_bucket_unlock(const struct qht_map *map,
> +                                     struct qht_bucket *b)
> +{
> +    qht_bucket_lock_do(map, b, qemu_spin_unlock);
> +}
> +
> +static inline void qht_head_init(struct qht_map *map, struct qht_bucket *b)
>  {
>      memset(b, 0, sizeof(*b));
> -    qemu_spin_init(&b->lock);
> +    qht_do_if_first_in_stripe(map, b, qemu_spin_init);
>      seqlock_init(&b->sequence);
>  }
>  
> @@ -250,7 +322,7 @@ static void qht_map_lock_buckets(struct qht_map *map)
>      for (i = 0; i < map->n_buckets; i++) {
>          struct qht_bucket *b = &map->buckets[i];
>  
> -        qemu_spin_lock(&b->lock);
> +        qht_do_if_first_in_stripe(map, b, qemu_spin_lock);
>      }
>  }
>  
> @@ -261,7 +333,7 @@ static void qht_map_unlock_buckets(struct qht_map *map)
>      for (i = 0; i < map->n_buckets; i++) {
>          struct qht_bucket *b = &map->buckets[i];
>  
> -        qemu_spin_unlock(&b->lock);
> +        qht_do_if_first_in_stripe(map, b, qemu_spin_unlock);
>      }
>  }
>  
> @@ -308,7 +380,7 @@ void qht_map_lock_buckets__no_stale(struct qht *ht, 
> struct qht_map **pmap)
>   * Get a head bucket and lock it, making sure its parent map is not stale.
>   * @pmap is filled with a pointer to the bucket's parent map.
>   *
> - * Unlock with qemu_spin_unlock(&b->lock).
> + * Unlock with qht_bucket_unlock.
>   *
>   * Note: callers cannot have ht->lock held.
>   */
> @@ -322,18 +394,18 @@ struct qht_bucket *qht_bucket_lock__no_stale(struct qht 
> *ht, uint32_t hash,
>      map = qatomic_rcu_read(&ht->map);
>      b = qht_map_to_bucket(map, hash);
>  
> -    qemu_spin_lock(&b->lock);
> +    qht_bucket_lock(map, b);
>      if (likely(!qht_map_is_stale__locked(ht, map))) {
>          *pmap = map;
>          return b;
>      }
> -    qemu_spin_unlock(&b->lock);
> +    qht_bucket_unlock(map, b);
>  
>      /* we raced with a resize; acquire ht->lock to see the updated ht->map */
>      qht_lock(ht);
>      map = ht->map;
>      b = qht_map_to_bucket(map, hash);
> -    qemu_spin_lock(&b->lock);
> +    qht_bucket_lock(map, b);
>      qht_unlock(ht);
>      *pmap = map;
>      return b;
> @@ -345,12 +417,13 @@ static inline bool qht_map_needs_resize(const struct 
> qht_map *map)
>             map->n_added_buckets_threshold;
>  }
>  
> -static inline void qht_chain_destroy(const struct qht_bucket *head)
> +static inline void qht_chain_destroy(const struct qht_map *map,
> +                                     struct qht_bucket *head)
>  {
>      struct qht_bucket *curr = head->next;
>      struct qht_bucket *prev;
>  
> -    qemu_spin_destroy(&head->lock);
> +    qht_do_if_first_in_stripe(map, head, qemu_spin_destroy);
>      while (curr) {
>          prev = curr;
>          curr = curr->next;
> @@ -364,7 +437,7 @@ static void qht_map_destroy(struct qht_map *map)
>      size_t i;
>  
>      for (i = 0; i < map->n_buckets; i++) {
> -        qht_chain_destroy(&map->buckets[i]);
> +        qht_chain_destroy(map, &map->buckets[i]);
>      }
>      qemu_vfree(map->buckets);
>      g_free(map);
> @@ -390,7 +463,7 @@ static struct qht_map *qht_map_create(size_t n_buckets)
>      map->buckets = qemu_memalign(QHT_BUCKET_ALIGN,
>                                   sizeof(*map->buckets) * n_buckets);
>      for (i = 0; i < n_buckets; i++) {
> -        qht_head_init(&map->buckets[i]);
> +        qht_head_init(map, &map->buckets[i]);
>      }
>      return map;
>  }
> @@ -638,7 +711,7 @@ bool qht_insert(struct qht *ht, void *p, uint32_t hash, 
> void **existing)
>      b = qht_bucket_lock__no_stale(ht, hash, &map);
>      prev = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
>      qht_bucket_debug__locked(b);
> -    qemu_spin_unlock(&b->lock);
> +    qht_bucket_unlock(map, b);
>  
>      if (unlikely(needs_resize) && ht->mode & QHT_MODE_AUTO_RESIZE) {
>          qht_grow_maybe(ht);
> @@ -749,7 +822,7 @@ bool qht_remove(struct qht *ht, const void *p, uint32_t 
> hash)
>      b = qht_bucket_lock__no_stale(ht, hash, &map);
>      ret = qht_remove__locked(b, p, hash);
>      qht_bucket_debug__locked(b);
> -    qemu_spin_unlock(&b->lock);
> +    qht_bucket_unlock(map, b);
>      return ret;
>  }


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro