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Subject[PATCH] relay: fix timer madness
From: Ingo Molnar <mingo@elte.hu>

When I'm using ktap script to tracing all event tracepoints by relay
transport, without this patch, the system will hang in few seconds.

I found the original patch discussion in 2007.
http://marc.info/?l=linux-kernel&m=118544794717162&w=2
(In that mail thread, the patch didn't fix that problem, but it fix
the problem I encountered now)

Changed from origina patch:
- mod timer interval changed from jiffies+1 to HZ/10, as Ingo suggested.
- mod timer interval changed from HZ/10 to jiffies + HZ/10, suggested
by Dan Carpenter, since mod_timer() takes an offset for interval.

Original patch changelog from Ingo in 2007:

Remove timer calls (!!!) from deep within the tracing infrastructure.
This was totally bogus code that can cause lockups and worse.
Poll the buffer every 2 jiffies for now.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: "zhangwei(Jovi)" <jovi.zhangwei@huawei.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Dumazet <edumazet@google.com>
---
kernel/relay.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index b91488b..42d6de3 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -339,6 +339,10 @@ static void wakeup_readers(unsigned long data)
{
struct rchan_buf *buf = (struct rchan_buf *)data;
wake_up_interruptible(&buf->read_wait);
+ /*
+ * Stupid polling for now:
+ */
+ mod_timer(&buf->timer, jiffies + HZ / 10);
}

/**
@@ -356,6 +360,7 @@ static void __relay_reset(struct rchan_buf *buf, unsigned int init)
init_waitqueue_head(&buf->read_wait);
kref_init(&buf->kref);
setup_timer(&buf->timer, wakeup_readers, (unsigned long)buf);
+ mod_timer(&buf->timer, jiffies + HZ / 10);
} else
del_timer_sync(&buf->timer);

@@ -739,15 +744,6 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
else
buf->early_bytes += buf->chan->subbuf_size -
buf->padding[old_subbuf];
- smp_mb();
- if (waitqueue_active(&buf->read_wait))
- /*
- * Calling wake_up_interruptible() from here
- * will deadlock if we happen to be logging
- * from the scheduler (trying to re-grab
- * rq->lock), so defer it.
- */
- mod_timer(&buf->timer, jiffies + 1);
}

old = buf->data;
--
1.7.9.7



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