cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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rwbase_rt.c (8384B)


      1// SPDX-License-Identifier: GPL-2.0-only
      2
      3/*
      4 * RT-specific reader/writer semaphores and reader/writer locks
      5 *
      6 * down_write/write_lock()
      7 *  1) Lock rtmutex
      8 *  2) Remove the reader BIAS to force readers into the slow path
      9 *  3) Wait until all readers have left the critical section
     10 *  4) Mark it write locked
     11 *
     12 * up_write/write_unlock()
     13 *  1) Remove the write locked marker
     14 *  2) Set the reader BIAS, so readers can use the fast path again
     15 *  3) Unlock rtmutex, to release blocked readers
     16 *
     17 * down_read/read_lock()
     18 *  1) Try fast path acquisition (reader BIAS is set)
     19 *  2) Take tmutex::wait_lock, which protects the writelocked flag
     20 *  3) If !writelocked, acquire it for read
     21 *  4) If writelocked, block on tmutex
     22 *  5) unlock rtmutex, goto 1)
     23 *
     24 * up_read/read_unlock()
     25 *  1) Try fast path release (reader count != 1)
     26 *  2) Wake the writer waiting in down_write()/write_lock() #3
     27 *
     28 * down_read/read_lock()#3 has the consequence, that rw semaphores and rw
     29 * locks on RT are not writer fair, but writers, which should be avoided in
     30 * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
     31 * inheritance mechanism.
     32 *
     33 * It's possible to make the rw primitives writer fair by keeping a list of
     34 * active readers. A blocked writer would force all newly incoming readers
     35 * to block on the rtmutex, but the rtmutex would have to be proxy locked
     36 * for one reader after the other. We can't use multi-reader inheritance
     37 * because there is no way to support that with SCHED_DEADLINE.
     38 * Implementing the one by one reader boosting/handover mechanism is a
     39 * major surgery for a very dubious value.
     40 *
     41 * The risk of writer starvation is there, but the pathological use cases
     42 * which trigger it are not necessarily the typical RT workloads.
     43 *
     44 * Fast-path orderings:
     45 * The lock/unlock of readers can run in fast paths: lock and unlock are only
     46 * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
     47 * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
     48 * and _release() (or stronger).
     49 *
     50 * Common code shared between RT rw_semaphore and rwlock
     51 */
     52
     53static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
     54{
     55	int r;
     56
     57	/*
     58	 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
     59	 * set.
     60	 */
     61	for (r = atomic_read(&rwb->readers); r < 0;) {
     62		if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))
     63			return 1;
     64	}
     65	return 0;
     66}
     67
     68static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
     69				      unsigned int state)
     70{
     71	struct rt_mutex_base *rtm = &rwb->rtmutex;
     72	int ret;
     73
     74	raw_spin_lock_irq(&rtm->wait_lock);
     75	/*
     76	 * Allow readers, as long as the writer has not completely
     77	 * acquired the semaphore for write.
     78	 */
     79	if (atomic_read(&rwb->readers) != WRITER_BIAS) {
     80		atomic_inc(&rwb->readers);
     81		raw_spin_unlock_irq(&rtm->wait_lock);
     82		return 0;
     83	}
     84
     85	/*
     86	 * Call into the slow lock path with the rtmutex->wait_lock
     87	 * held, so this can't result in the following race:
     88	 *
     89	 * Reader1		Reader2		Writer
     90	 *			down_read()
     91	 *					down_write()
     92	 *					rtmutex_lock(m)
     93	 *					wait()
     94	 * down_read()
     95	 * unlock(m->wait_lock)
     96	 *			up_read()
     97	 *			wake(Writer)
     98	 *					lock(m->wait_lock)
     99	 *					sem->writelocked=true
    100	 *					unlock(m->wait_lock)
    101	 *
    102	 *					up_write()
    103	 *					sem->writelocked=false
    104	 *					rtmutex_unlock(m)
    105	 *			down_read()
    106	 *					down_write()
    107	 *					rtmutex_lock(m)
    108	 *					wait()
    109	 * rtmutex_lock(m)
    110	 *
    111	 * That would put Reader1 behind the writer waiting on
    112	 * Reader2 to call up_read(), which might be unbound.
    113	 */
    114
    115	trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);
    116
    117	/*
    118	 * For rwlocks this returns 0 unconditionally, so the below
    119	 * !ret conditionals are optimized out.
    120	 */
    121	ret = rwbase_rtmutex_slowlock_locked(rtm, state);
    122
    123	/*
    124	 * On success the rtmutex is held, so there can't be a writer
    125	 * active. Increment the reader count and immediately drop the
    126	 * rtmutex again.
    127	 *
    128	 * rtmutex->wait_lock has to be unlocked in any case of course.
    129	 */
    130	if (!ret)
    131		atomic_inc(&rwb->readers);
    132	raw_spin_unlock_irq(&rtm->wait_lock);
    133	if (!ret)
    134		rwbase_rtmutex_unlock(rtm);
    135
    136	trace_contention_end(rwb, ret);
    137	return ret;
    138}
    139
    140static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
    141					    unsigned int state)
    142{
    143	if (rwbase_read_trylock(rwb))
    144		return 0;
    145
    146	return __rwbase_read_lock(rwb, state);
    147}
    148
    149static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
    150					 unsigned int state)
    151{
    152	struct rt_mutex_base *rtm = &rwb->rtmutex;
    153	struct task_struct *owner;
    154	DEFINE_RT_WAKE_Q(wqh);
    155
    156	raw_spin_lock_irq(&rtm->wait_lock);
    157	/*
    158	 * Wake the writer, i.e. the rtmutex owner. It might release the
    159	 * rtmutex concurrently in the fast path (due to a signal), but to
    160	 * clean up rwb->readers it needs to acquire rtm->wait_lock. The
    161	 * worst case which can happen is a spurious wakeup.
    162	 */
    163	owner = rt_mutex_owner(rtm);
    164	if (owner)
    165		rt_mutex_wake_q_add_task(&wqh, owner, state);
    166
    167	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
    168	preempt_disable();
    169	raw_spin_unlock_irq(&rtm->wait_lock);
    170	rt_mutex_wake_up_q(&wqh);
    171}
    172
    173static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
    174					       unsigned int state)
    175{
    176	/*
    177	 * rwb->readers can only hit 0 when a writer is waiting for the
    178	 * active readers to leave the critical section.
    179	 *
    180	 * dec_and_test() is fully ordered, provides RELEASE.
    181	 */
    182	if (unlikely(atomic_dec_and_test(&rwb->readers)))
    183		__rwbase_read_unlock(rwb, state);
    184}
    185
    186static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
    187					 unsigned long flags)
    188{
    189	struct rt_mutex_base *rtm = &rwb->rtmutex;
    190
    191	/*
    192	 * _release() is needed in case that reader is in fast path, pairing
    193	 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
    194	 */
    195	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
    196	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
    197	rwbase_rtmutex_unlock(rtm);
    198}
    199
    200static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
    201{
    202	struct rt_mutex_base *rtm = &rwb->rtmutex;
    203	unsigned long flags;
    204
    205	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
    206	__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
    207}
    208
    209static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
    210{
    211	struct rt_mutex_base *rtm = &rwb->rtmutex;
    212	unsigned long flags;
    213
    214	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
    215	/* Release it and account current as reader */
    216	__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
    217}
    218
    219static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
    220{
    221	/* Can do without CAS because we're serialized by wait_lock. */
    222	lockdep_assert_held(&rwb->rtmutex.wait_lock);
    223
    224	/*
    225	 * _acquire is needed in case the reader is in the fast path, pairing
    226	 * with rwbase_read_unlock(), provides ACQUIRE.
    227	 */
    228	if (!atomic_read_acquire(&rwb->readers)) {
    229		atomic_set(&rwb->readers, WRITER_BIAS);
    230		return 1;
    231	}
    232
    233	return 0;
    234}
    235
    236static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
    237				     unsigned int state)
    238{
    239	struct rt_mutex_base *rtm = &rwb->rtmutex;
    240	unsigned long flags;
    241
    242	/* Take the rtmutex as a first step */
    243	if (rwbase_rtmutex_lock_state(rtm, state))
    244		return -EINTR;
    245
    246	/* Force readers into slow path */
    247	atomic_sub(READER_BIAS, &rwb->readers);
    248
    249	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
    250	if (__rwbase_write_trylock(rwb))
    251		goto out_unlock;
    252
    253	rwbase_set_and_save_current_state(state);
    254	trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
    255	for (;;) {
    256		/* Optimized out for rwlocks */
    257		if (rwbase_signal_pending_state(state, current)) {
    258			rwbase_restore_current_state();
    259			__rwbase_write_unlock(rwb, 0, flags);
    260			trace_contention_end(rwb, -EINTR);
    261			return -EINTR;
    262		}
    263
    264		if (__rwbase_write_trylock(rwb))
    265			break;
    266
    267		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
    268		rwbase_schedule();
    269		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
    270
    271		set_current_state(state);
    272	}
    273	rwbase_restore_current_state();
    274	trace_contention_end(rwb, 0);
    275
    276out_unlock:
    277	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
    278	return 0;
    279}
    280
    281static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
    282{
    283	struct rt_mutex_base *rtm = &rwb->rtmutex;
    284	unsigned long flags;
    285
    286	if (!rwbase_rtmutex_trylock(rtm))
    287		return 0;
    288
    289	atomic_sub(READER_BIAS, &rwb->readers);
    290
    291	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
    292	if (__rwbase_write_trylock(rwb)) {
    293		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
    294		return 1;
    295	}
    296	__rwbase_write_unlock(rwb, 0, flags);
    297	return 0;
    298}