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Applications use -ECONNREFUSED as returned from write() in order to
determine that a socket should be closed. However, when using connected
dgram unix sockets in a poll/write loop, a final POLLOUT event can be
missed when the remote end closes. Thus, the poll is stuck forever:
thread 1 (client) thread 2 (server)
connect() to server
write() returns -EAGAIN
unix_dgram_poll()
-> unix_recvq_full() is true
close()
->unix_release_sock()
->wake_up_interruptible_all()
unix_dgram_poll() (due to the
wake_up_interruptible_all)
-> unix_recvq_full() still is true
->free all skbs
Now thread 1 is stuck and will not receive anymore wakeups. In this
case, when thread 1 gets the -EAGAIN, it has not queued any skbs
otherwise the 'free all skbs' step would in fact cause a wakeup and
a POLLOUT return. So the race here is probably fairly rare because
it means there are no skbs that thread 1 queued and that thread 1
schedules before the 'free all skbs' step.
This issue was reported as a hang when /dev/log is closed.
The fix is to signal POLLOUT if the socket is marked as SOCK_DEAD, which
means a subsequent write() will get -ECONNREFUSED.
Reported-by: Ian Lance Taylor <iant@golang.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.