Messages in this thread Patch in this message | | | Date | Wed, 8 Sep 2021 16:49:20 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader |
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On Wed, Sep 08, 2021 at 04:41:10PM +0200, Peter Zijlstra wrote:
> Subject: sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() > > While looking at current_save_and_set_rtlock_wait_state() I'm thinking > it really ought to use smp_store_mb(), because something like: > > current_save_and_set_rtlock_wait_state(); > for (;;) { > if (try_lock()) > break; > raw_spin_unlock_irq(&lock->wait_lock); > if (!cond) > schedule(); > raw_spin_lock_irq(&lock->wait_lock); > set_current_state(TASK_RTLOCK_WAIT); > } > current_restore_rtlock_saved_state(); > > which is very close to the advertised usage in the comment, is actually > broken I think: > > - try_lock() doesn't need to provide any ordering on failure; > - raw_spin_unlock() only needs to provide RELEASE ordering; > > which gives that the above turns into something like: > > WRITE_ONCE(current->__state, TASK_RTLOCK_WAIT); > raw_spin_unlock(¤t->pi_lock); > raw_spin_unlock(&lock->wait_lock); > if (!cond) > > and the load of @cond is then allowed to speculate right before the > __state store, and we've got a missed wakeup -> BAD(tm). > > Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks") > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > ---
On top of which we can do your patch like.
--- Subject: lockin/rwbase: Take care of ordering guarantee for fastpath reader From: Boqun Feng <boqun.feng@gmail.com> Date: Wed, 1 Sep 2021 23:06:27 +0800
From: Boqun Feng <boqun.feng@gmail.com>
Readers of rwbase can lock and unlock without taking any inner lock, if that happens, we need the ordering provided by atomic operations to satisfy the ordering semantics of lock/unlock. Without that, considering the follow case:
{ X = 0 initially }
CPU 0 CPU 1 ===== ===== rt_write_lock(); X = 1 rt_write_unlock(): atomic_add(READER_BIAS - WRITER_BIAS, ->readers); // ->readers is READER_BIAS. rt_read_lock(): if ((r = atomic_read(->readers)) < 0) // True atomic_try_cmpxchg(->readers, r, r + 1); // succeed. <acquire the read lock via fast path>
r1 = X; // r1 may be 0, because nothing prevent the reordering // of "X=1" and atomic_add() on CPU 1.
Therefore audit every usage of atomic operations that may happen in a fast path, and add necessary barriers.
Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20210901150627.620830-1-boqun.feng@gmail.com --- kernel/locking/rwbase_rt.c | 41 ++++++++++++++++++++++++++++++++++++----- 1 file changed, 36 insertions(+), 5 deletions(-)
--- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -41,6 +41,12 @@ * The risk of writer starvation is there, but the pathological use cases * which trigger it are not necessarily the typical RT workloads. * + * Fast-path orderings: + * The lock/unlock of readers can run in fast paths: lock and unlock are only + * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE + * semantics of rwbase_rt. Atomic ops then should be stronger than _acquire() + * and _release() to provide necessary ordering guarantee. + * * Common code shared between RT rw_semaphore and rwlock */ @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_t * set. */ for (r = atomic_read(&rwb->readers); r < 0;) { + /* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */ if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1))) return 1; } @@ -162,6 +169,8 @@ static __always_inline void rwbase_read_ /* * rwb->readers can only hit 0 when a writer is waiting for the * active readers to leave the critical section. + * + * dec_and_test() is fully ordered, provides RELEASE. */ if (unlikely(atomic_dec_and_test(&rwb->readers))) __rwbase_read_unlock(rwb, state); @@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock { struct rt_mutex_base *rtm = &rwb->rtmutex; - atomic_add(READER_BIAS - bias, &rwb->readers); + /* + * _release() is needed in case that reader is in fast path, pairing + * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE + */ + (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers); raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); rwbase_rtmutex_unlock(rtm); } @@ -201,6 +214,7 @@ static int __sched rwbase_write_lock(str { struct rt_mutex_base *rtm = &rwb->rtmutex; unsigned long flags; + int readers; /* Take the rtmutex as a first step */ if (rwbase_rtmutex_lock_state(rtm, state)) @@ -210,14 +224,23 @@ static int __sched rwbase_write_lock(str atomic_sub(READER_BIAS, &rwb->readers); raw_spin_lock_irqsave(&rtm->wait_lock, flags); + + /* The below set_*_state() thingy implies smp_mb() to provide ACQUIRE */ + readers = atomic_read(&rwb->readers); /* * set_current_state() for rw_semaphore * current_save_and_set_rtlock_wait_state() for rwlock */ rwbase_set_and_save_current_state(state); - /* Block until all readers have left the critical section. */ - for (; atomic_read(&rwb->readers);) { + /* + * Block until all readers have left the critical section. + * + * _acqurie() is needed in case that the reader side runs in the fast + * path, pairing with the atomic_dec_and_test() in rwbase_read_unlock(), + * provides ACQUIRE. + */ + while (readers) { /* Optimized out for rwlocks */ if (rwbase_signal_pending_state(state, current)) { __set_current_state(TASK_RUNNING); @@ -229,8 +252,12 @@ static int __sched rwbase_write_lock(str /* * Schedule and wait for the readers to leave the critical * section. The last reader leaving it wakes the waiter. + * + * _acquire() is not needed, because we can rely on the smp_mb() + * in set_current_state() to provide ACQUIRE. */ - if (atomic_read(&rwb->readers) != 0) + readers = atomic_read(&rwb->readers); + if (readers) rwbase_schedule(); set_current_state(state); raw_spin_lock_irqsave(&rtm->wait_lock, flags); @@ -253,7 +280,11 @@ static inline int rwbase_write_trylock(s atomic_sub(READER_BIAS, &rwb->readers); raw_spin_lock_irqsave(&rtm->wait_lock, flags); - if (!atomic_read(&rwb->readers)) { + /* + * _acquire() is needed in case reader is in the fast path, pairing with + * rwbase_read_unlock(), provides ACQUIRE. + */ + if (!atomic_read_acquire(&rwb->readers)) { atomic_set(&rwb->readers, WRITER_BIAS); raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); return 1;
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