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01abd02508
There is a very difficult to reproduce race in the listener's accept
code, which is much easier to reproduce once connection limits are
properly enforced. It's an ABBA lock issue :
- the following functions take l->lock then lq_lock :
disable_listener, pause_listener, listener_full, limit_listener,
do_unbind_listener
- the following ones take lq_lock then l->lock :
resume_listener, dequeue_all_listener
This is because __resume_listener() only takes the listener's lock
and expects to be called with lq_lock held. The problem can easily
happen when listener_full() and limit_listener() are called a lot
while in parallel another thread releases sessions for the same
listener using listener_release() which in turn calls resume_listener().
This scenario is more prevalent in 2.0-dev since the removal of the
accept lock in listener_accept(). However in 1.9 and before, a different
but extremely unlikely scenario can happen :
thread1 thread2
............................ enter listener_accept()
limit_listener()
............................ long pause before taking the lock
session_free()
dequeue_all_listeners()
lock(lq_lock) [1]
............................ try_lock(l->lock) [2]
__resume_listener()
spin_lock(l->lock) =>WAIT[2]
............................ accept()
l->accept()
nbconn==maxconn =>
listener_full()
state==LI_LIMITED =>
lock(lq_lock) =>DEADLOCK[1]!
In practice it is almost impossible to trigger it because it requires
to limit both on the listener's maxconn and the frontend's rate limit,
at the same time, and to release the listener when the connection rate
goes below the limit between poll() returns the FD and the lock is
taken (a few nanoseconds). But maybe with threads competing on the
same core it has more chances to appear.
This patch removes the lq_lock and replaces it with a lockless queue
for the listener's wait queue (well, technically speaking a self-locked
queue) brought by commit
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| .. | ||
| accept4.h | ||
| base64.h | ||
| buf.h | ||
| buffer.h | ||
| cfgparse.h | ||
| chunk.h | ||
| compat.h | ||
| compiler.h | ||
| config.h | ||
| debug.h | ||
| defaults.h | ||
| epoll.h | ||
| errors.h | ||
| h1.h | ||
| h2.h | ||
| hash.h | ||
| hathreads.h | ||
| hpack-dec.h | ||
| hpack-enc.h | ||
| hpack-huff.h | ||
| hpack-tbl.h | ||
| http-hdr.h | ||
| http.h | ||
| htx.h | ||
| initcall.h | ||
| ist.h | ||
| istbuf.h | ||
| memory.h | ||
| mini-clist.h | ||
| namespace.h | ||
| net_helper.h | ||
| regex.h | ||
| splice.h | ||
| standard.h | ||
| syscall.h | ||
| template.h | ||
| ticks.h | ||
| time.h | ||
| tools.h | ||
| uri_auth.h | ||
| version.h | ||
| xref.h | ||