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fs/epoll: make nesting accounting safe for -rt kernel 

Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set
ep_poll_safewake() can take several non-raw spinlocks after disabling
interrupts.  Since a spinlock can block in the -rt kernel, we can't take a
spinlock after disabling interrupts.  So let's re-work how we determine
the nesting level such that it plays nicely with the -rt kernel.

Let's introduce a 'nests' field in struct eventpoll that records the
current nesting level during ep_poll_callback().  Then, if we nest again
we can find the previous struct eventpoll that we were called from and
increase our count by 1.  The 'nests' field is protected by
ep->poll_wait.lock.

I've also moved the visited field to reduce the size of struct eventpoll
from 184 bytes to 176 bytes on x86_64 for !CONFIG_DEBUG_LOCK_ALLOC, which
is typical for a production config.

Reported-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Roman Penyaev <rpenyaev@suse.de>
Cc: Eric Wong <normalperson@yhbt.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/1582739816-13167-1-git-send-email-jbaron@akamai.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jason Baron 2020-04-06 20:11:23 -07:00 committed by Linus Torvalds
parent 282144e04b
commit efcdd350d1
1 changed files with 43 additions and 21 deletions
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64
fs/eventpoll.c
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@ -218,13 +218,18 @@ struct eventpoll {
struct file *file;
/* used to optimize loop detection check */
int visited;
struct list_head visited_list_link;
int visited;
#ifdef CONFIG_NET_RX_BUSY_POLL
/* used to track busy poll napi_id */
unsigned int napi_id;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/* tracks wakeup nests for lockdep validation */
u8 nests;
#endif
};
/* Wait structure used by the poll hooks */
@ -545,30 +550,47 @@ out_unlock:
*/
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static DEFINE_PER_CPU(int, wakeup_nest);
static void ep_poll_safewake(wait_queue_head_t *wq)
static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
{
struct eventpoll *ep_src;
unsigned long flags;
int subclass;
u8 nests = 0;
local_irq_save(flags);
preempt_disable();
subclass = __this_cpu_read(wakeup_nest);
spin_lock_nested(&wq->lock, subclass + 1);
__this_cpu_inc(wakeup_nest);
wake_up_locked_poll(wq, POLLIN);
__this_cpu_dec(wakeup_nest);
spin_unlock(&wq->lock);
local_irq_restore(flags);
preempt_enable();
/*
* To set the subclass or nesting level for spin_lock_irqsave_nested()
* it might be natural to create a per-cpu nest count. However, since
* we can recurse on ep->poll_wait.lock, and a non-raw spinlock can
* schedule() in the -rt kernel, the per-cpu variable are no longer
* protected. Thus, we are introducing a per eventpoll nest field.
* If we are not being call from ep_poll_callback(), epi is NULL and
* we are at the first level of nesting, 0. Otherwise, we are being
* called from ep_poll_callback() and if a previous wakeup source is
* not an epoll file itself, we are at depth 1 since the wakeup source
* is depth 0. If the wakeup source is a previous epoll file in the
* wakeup chain then we use its nests value and record ours as
* nests + 1. The previous epoll file nests value is stable since its
* already holding its own poll_wait.lock.
*/
if (epi) {
if ((is_file_epoll(epi->ffd.file))) {
ep_src = epi->ffd.file->private_data;
nests = ep_src->nests;
} else {
nests = 1;
}
}
spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
ep->nests = nests + 1;
wake_up_locked_poll(&ep->poll_wait, EPOLLIN);
ep->nests = 0;
spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
}
#else
static void ep_poll_safewake(wait_queue_head_t *wq)
static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
{
wake_up_poll(wq, EPOLLIN);
wake_up_poll(&ep->poll_wait, EPOLLIN);
}
#endif
@ -789,7 +811,7 @@ static void ep_free(struct eventpoll *ep)
/* We need to release all tasks waiting for these file */
if (waitqueue_active(&ep->poll_wait))
ep_poll_safewake(&ep->poll_wait);
ep_poll_safewake(ep, NULL);
/*
* We need to lock this because we could be hit by
@ -1258,7 +1280,7 @@ out_unlock:
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&ep->poll_wait);
ep_poll_safewake(ep, epi);
if (!(epi->event.events & EPOLLEXCLUSIVE))
ewake = 1;
@ -1562,7 +1584,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&ep->poll_wait);
ep_poll_safewake(ep, NULL);
return 0;
@ -1666,7 +1688,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&ep->poll_wait);
ep_poll_safewake(ep, NULL);
return 0;
}