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Alexei Starovoitov a35a76faad Merge branch 'fix_bpf_send_signal' 
Yonghong Song says:

====================
Commit 8b401f9ed2 ("bpf: implement bpf_send_signal() helper")
introduced bpf_send_signal() helper and Commit 8482941f09
("bpf: Add bpf_send_signal_thread() helper") added bpf_send_signal_thread()
helper. Both helpers try to send a signel to current process or thread.

When bpf_prog is called with scheduler rq_lock held, a deadlock
could happen since bpf_send_signal() and bpf_send_signal_thread()
will call group_send_sig_info() which may ultimately want to acquire
rq_lock() again. This happens in 5.2 and 4.16 based kernels in our
production environment with perf_sw_event.

In a different scenario, the following is also possible in the last kernel:
  cpu 1:
     do_task_stat <- holding sighand->siglock
     ...
     task_sched_runtime <- trying to grab rq_lock

  cpu 2:
     __schedule <- holding rq_lock
     ...
     do_send_sig_info <- trying to grab sighand->siglock

Commit eac9153f2b ("bpf/stackmap: Fix deadlock with
rq_lock in bpf_get_stack()") has a similar issue with above
rq_lock() deadlock. This patch set addressed the issue
in a similar way. Patch #1 provided kernel solution and
Patch #2 added a selftest.

Changelogs:
  v2 -> v3:
    . simplify selftest send_signal_sched_switch().
      The previous version has mmap/munmap inherited
      from Song's reproducer. They are not necessary
      in this context.
  v1 -> v2:
    . previous fix using task_work in nmi() is incorrect.
      there is no nmi() involvement here. Using task_work
      in all cases might be a solution. But decided to
      use a similar fix as in Commit eac9153f2b.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
History 2020-03-05 14:09:22 -08:00
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README 

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.

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.