[PATCH 6/7] sched: Clean up preempt_enable_no_resched() abuse

[Peter Zijlstra]

The only valid use of preempt_enable_no_resched() is if the very next
line is schedule() or if we know preemption cannot actually be enabled
by that statement due to known more preempt_count 'refs'.

As to the busy_poll mess; that looks to be completely and utterly
broken, sched_clock() can return utter garbage with interrupts enabled
(rare but still), it can drift unbounded between CPUs, so if you get
preempted/migrated and your new CPU is years behind on the previous
CPU we get to busy spin for a _very_ long time. There is a _REASON_
sched_clock() warns about preemptability - papering over it with a
preempt_disable()/preempt_enable_no_resched() is just terminal brain
damage on so many levels.

Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: lenb@xxxxxxxxxx
Cc: rjw@xxxxxxxxxxxxx
Cc: Eliezer Tamir <eliezer.tamir@xxxxxxxxxxxxxxx>
Cc: Chris Leech <christopher.leech@xxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: rui.zhang@xxxxxxxxx
Cc: jacob.jun.pan@xxxxxxxxxxxxxxx
Cc: Mike Galbraith <bitbucket@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: hpa@xxxxxxxxx
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 include/net/busy_poll.h |   20 ++++++++------------
 net/ipv4/tcp.c          |    4 ++--
 2 files changed, 10 insertions(+), 14 deletions(-)

--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -42,27 +42,23 @@ static inline bool net_busy_loop_on(void
 	return sysctl_net_busy_poll;
 }
 
-/* a wrapper to make debug_smp_processor_id() happy
- * we can use sched_clock() because we don't care much about precision
- * we only care that the average is bounded
- */
-#ifdef CONFIG_DEBUG_PREEMPT
 static inline u64 busy_loop_us_clock(void)
 {
 	u64 rc;
 
+	/*
+	 * XXX with interrupts enabled sched_clock() can return utter garbage
+	 * Futhermore, it can have unbounded drift between CPUs, so the below
+	 * usage is terminally broken and only serves to shut up a valid debug
+	 * warning.
+	 */
+
 	preempt_disable_notrace();
 	rc = sched_clock();
-	preempt_enable_no_resched_notrace();
+	preempt_enable_notrace();
 
 	return rc >> 10;
 }
-#else /* CONFIG_DEBUG_PREEMPT */
-static inline u64 busy_loop_us_clock(void)
-{
-	return sched_clock() >> 10;
-}
-#endif /* CONFIG_DEBUG_PREEMPT */
 
 static inline unsigned long sk_busy_loop_end_time(struct sock *sk)
 {
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1623,11 +1623,11 @@ int tcp_recvmsg(struct kiocb *iocb, stru
 		    (len > sysctl_tcp_dma_copybreak) && !(flags & MSG_PEEK) &&
 		    !sysctl_tcp_low_latency &&
 		    net_dma_find_channel()) {
-			preempt_enable_no_resched();
+			preempt_enable();
 			tp->ucopy.pinned_list =
 					dma_pin_iovec_pages(msg->msg_iov, len);
 		} else {
-			preempt_enable_no_resched();
+			preempt_enable();
 		}
 	}
 #endif


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