lkml.org 
[lkml]   [2019]   [Apr]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation
On Sat, Apr 13, 2019 at 01:22:50PM -0400, Waiman Long wrote:

> +#define RWSEM_COUNT_HANDOFF(c) ((c) & RWSEM_FLAG_HANDOFF)
> +#define RWSEM_COUNT_LOCKED_OR_HANDOFF(c) \
> + ((c) & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))

Like said before, I also made these go away.

> @@ -245,6 +274,8 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> struct rwsem_waiter *waiter, *tmp;
> long oldcount, woken = 0, adjustment = 0;
>
> + lockdep_assert_held(&sem->wait_lock);
> +
> /*
> * Take a peek at the queue head waiter such that we can determine
> * the wakeup(s) to perform.
> @@ -276,6 +307,15 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> adjustment = RWSEM_READER_BIAS;
> oldcount = atomic_long_fetch_add(adjustment, &sem->count);
> if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
> + /*
> + * Initiate handoff to reader, if applicable.
> + */
> + if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
> + time_after(jiffies, waiter->timeout)) {
> + adjustment -= RWSEM_FLAG_HANDOFF;
> + lockevent_inc(rwsem_rlock_handoff);
> + }

/*
* When we've been waiting 'too' long (for
* writers to give up the lock) request a
* HANDOFF to force the issue.
*/

?

> +
> atomic_long_sub(adjustment, &sem->count);

Can we change this to: atomic_long_add() please? The below loop that
wakes all remaining readers does use add(), so it is a bit 'weird' to
have the adjustment being negated on handover.

> return;
> }
> @@ -324,6 +364,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> adjustment -= RWSEM_FLAG_WAITERS;
> }
>
> + /*
> + * Clear the handoff flag
> + */

Right, but that is a trivial comment in the 'increment i' style, it
clearly states what the code does, but completely fails to elucidate the
code.

Maybe:

/*
* When we've woken a reader, we no longer need to force writers
* to give up the lock and we can clear HANDOFF.
*/

And I suppose this is required if we were the pickup of the handoff set
above, but is there a guarantee that the HANDOFF was not set by a
writer?

> + if (woken && RWSEM_COUNT_HANDOFF(atomic_long_read(&sem->count)))
> + adjustment -= RWSEM_FLAG_HANDOFF;
> +
> if (adjustment)
> atomic_long_add(adjustment, &sem->count);
> }
> @@ -332,22 +378,42 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> * This function must be called with the sem->wait_lock held to prevent
> * race conditions between checking the rwsem wait list and setting the
> * sem->count accordingly.
> + *
> + * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
> + * bit is set or the lock is acquired.
> */
> +static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
> + enum writer_wait_state wstate)
> {
> long new;
>
lockdep_assert_held(&sem->wait_lock);

> +retry:
> + if (RWSEM_COUNT_LOCKED(count)) {
> + if (RWSEM_COUNT_HANDOFF(count) || (wstate != WRITER_HANDOFF))
> + return false;
> + /*
> + * The lock may become free just before setting handoff bit.
> + * It will be simpler if atomic_long_or_return() is available.
> + */
> + atomic_long_or(RWSEM_FLAG_HANDOFF, &sem->count);
> + count = atomic_long_read(&sem->count);
> + goto retry;
> + }
> +
> + if ((wstate == WRITER_NOT_FIRST) && RWSEM_COUNT_HANDOFF(count))
> return false;
>
> + new = (count & ~RWSEM_FLAG_HANDOFF) + RWSEM_WRITER_LOCKED -
> + (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0);
>
> if (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)) {
> rwsem_set_owner(sem);
> return true;
> }
>
> + if (unlikely((wstate == WRITER_HANDOFF) && !RWSEM_COUNT_HANDOFF(count)))
> + goto retry;
> +
> return false;
> }

This function gives me heartburn. Don't you just feel something readable
trying to struggle free from that?

See, if you first write that function in the form:

long new;

do {
new = count | RWSEM_WRITER_LOCKED;

if (count & RWSEM_LOCK_MASK)
return false;

if (list_is_singular(&sem->wait_list))
new &= ~RWSEM_FLAG_WAITERS;

} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

rwsem_set_owner(sem);
return true;

And then add the HANDOFF bits like:

long new;

do {
+ bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);

+ new = (count | RWSEM_WRITER_LOCKED) & ~RWSEM_FLAG_HANDOFF;

if (count & RWSEM_LOCK_MASK) {
+ if (has_handoff && wstate != WRITER_HANDOFF)
+ return false;
new |= RWSEM_FLAG_HANDOFF;
}

+ if (has_handoff && wstate == WRITER_NOT_FIRST)
+ return false;

if (list_is_singular(&sem->wait_list))
new &= ~RWSEM_FLAG_WAITERS;

} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

rwsem_set_owner(sem);
return true;

it almost looks like sensible code.

>
> @@ -359,7 +425,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> {
> long count = atomic_long_read(&sem->count);
>
> - while (!RWSEM_COUNT_LOCKED(count)) {
> + while (!RWSEM_COUNT_LOCKED_OR_HANDOFF(count)) {
> if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
> count + RWSEM_WRITER_LOCKED)) {

RWSEM_WRITER_LOCKED really should be RWSEM_FLAG_WRITER or something like
that, and since it is a flag, that really should've been | not +.

> rwsem_set_owner(sem);
> @@ -498,6 +564,16 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> }
> #endif
>
> +/*
> + * This is safe to be called without holding the wait_lock.
> + */
> +static inline bool
> +rwsem_waiter_is_first(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
> +{
> + return list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
> + == waiter;

Just bust the line limit on that, this is silly. If you feel strongly
about the 80 char thing, we could do:

#define rwsem_first_waiter(sem) \
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)

and use that in both locations. (and one could even write the
list_for_each_entry_safe() loop in the form:

while (!list_empty(&sem->wait_list)) {
entry = rwsem_first_waiter(sem);

...

list_del();
...
}
Although I suppose that gets you confused later on where you want to
wake more readers still... I'll get there,.. eventually.

> +}
> +
> /*
> * Wait for the read lock to be granted
> */
> @@ -510,16 +586,18 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
>
> waiter.task = current;
> waiter.type = RWSEM_WAITING_FOR_READ;
> + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
>
> raw_spin_lock_irq(&sem->wait_lock);
> if (list_empty(&sem->wait_list)) {
> /*
> * In case the wait queue is empty and the lock isn't owned
> + * by a writer or has the handoff bit set, this reader can
> + * exit the slowpath and return immediately as its
> + * RWSEM_READER_BIAS has already been set in the count.
> */
> + if (!(atomic_long_read(&sem->count) &
> + (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> raw_spin_unlock_irq(&sem->wait_lock);
> rwsem_set_reader_owned(sem);
> lockevent_inc(rwsem_rlock_fast);
> @@ -567,7 +645,8 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
> out_nolock:
> list_del(&waiter.list);
> if (list_empty(&sem->wait_list))
> + atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
> + &sem->count);

If you split the line, this wants { }.
> raw_spin_unlock_irq(&sem->wait_lock);
> __set_current_state(TASK_RUNNING);
> lockevent_inc(rwsem_rlock_fail);
> @@ -593,7 +672,7 @@ static inline struct rw_semaphore *
> __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> {
> long count;
> + enum writer_wait_state wstate;
> struct rwsem_waiter waiter;
> struct rw_semaphore *ret = sem;
> DEFINE_WAKE_Q(wake_q);
> @@ -608,56 +687,63 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> */
> waiter.task = current;
> waiter.type = RWSEM_WAITING_FOR_WRITE;
> + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
>
> raw_spin_lock_irq(&sem->wait_lock);
>
> /* account for this before adding a new element to the list */
> + wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
>
> list_add_tail(&waiter.list, &sem->wait_list);
>
> /* we're now waiting on the lock */
> + if (wstate == WRITER_NOT_FIRST) {
> count = atomic_long_read(&sem->count);
>
> /*
> + * If there were already threads queued before us and:
> + * 1) there are no no active locks, wake the front
> + * queued process(es) as the handoff bit might be set.
> + * 2) there are no active writers and some readers, the lock
> + * must be read owned; so we try to wake any read lock
> + * waiters that were queued ahead of us.
> */
> + if (!RWSEM_COUNT_LOCKED(count))
> + __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
> + else if (!(count & RWSEM_WRITER_MASK) &&
> + (count & RWSEM_READER_MASK))
> __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);

That RWSEM_WRITER_MASK is another layer of obfustaction we can do
without.

Does the above want to be something like:

if (!(count & RWSEM_WRITER_LOCKED)) {
__rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) ?
RWSEM_WAKE_READERS :
RWSEM_WAKE_ANY, &wake_q);
}
> + else
> + goto wait;
>
> + /*
> + * The wakeup is normally called _after_ the wait_lock
> + * is released, but given that we are proactively waking
> + * readers we can deal with the wake_q overhead as it is
> + * similar to releasing and taking the wait_lock again
> + * for attempting rwsem_try_write_lock().
> + */
> + wake_up_q(&wake_q);

Hurmph.. the reason we do wake_up_q() outside of wait_lock is such that
those tasks don't bounce on wait_lock. Also, it removes a great deal of
hold-time from wait_lock.

So I'm not sure I buy your argument here.

> + /*
> + * Reinitialize wake_q after use.
> + */

Or:
/* we need wake_q again below, reinitialize */
> + wake_q_init(&wake_q);
> } else {
> count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
> }
>
> +wait:
> /* wait until we successfully acquire the lock */
> set_current_state(state);
> while (true) {
> + if (rwsem_try_write_lock(count, sem, wstate))
> break;
> +
> raw_spin_unlock_irq(&sem->wait_lock);
>
> /* Block until there are no active lockers. */
> + for (;;) {
> if (signal_pending_state(state, current))
> goto out_nolock;
>
> @@ -665,9 +751,34 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> lockevent_inc(rwsem_sleep_writer);
> set_current_state(state);
> count = atomic_long_read(&sem->count);
> +
> + if ((wstate == WRITER_NOT_FIRST) &&
> + rwsem_waiter_is_first(sem, &waiter))
> + wstate = WRITER_FIRST;
> +
> + if (!RWSEM_COUNT_LOCKED(count))
> + break;
> +
> + /*
> + * An RT task sets the HANDOFF bit immediately.
> + * Non-RT task will wait a while before doing so.

Again, this describes what we already read the code to do; but doesn't
add anything.

> + *
> + * The setting of the handoff bit is deferred
> + * until rwsem_try_write_lock() is called.
> + */
> + if ((wstate == WRITER_FIRST) && (rt_task(current) ||
> + time_after(jiffies, waiter.timeout))) {
> + wstate = WRITER_HANDOFF;
> + lockevent_inc(rwsem_wlock_handoff);
> + /*
> + * Break out to call rwsem_try_write_lock().
> + */

Another exceedingly useful comment.

> + break;
> + }
> + }
>
> raw_spin_lock_irq(&sem->wait_lock);
> + count = atomic_long_read(&sem->count);
> }
> __set_current_state(TASK_RUNNING);
> list_del(&waiter.list);
> @@ -680,6 +791,12 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> __set_current_state(TASK_RUNNING);
> raw_spin_lock_irq(&sem->wait_lock);
> list_del(&waiter.list);
> + /*
> + * If handoff bit has been set by this waiter, make sure that the
> + * clearing of it is seen by others before proceeding.
> + */
> + if (unlikely(wstate == WRITER_HANDOFF))
> + atomic_long_add_return(-RWSEM_FLAG_HANDOFF, &sem->count);

_AGAIN_ no explanation what so ff'ing ever.

And why add_return() if you ignore the return value.

> if (list_empty(&sem->wait_list))
> atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);

And you could've easily combined the two flags in a single andnot op.

> else

\
 
 \ /
  Last update: 2019-04-16 17:50    [W:0.644 / U:4.812 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site