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From: "me at serhei dot io" <sourceware-bugzilla@sourceware.org>
To: systemtap@sourceware.org
Subject: [Bug bpf/24659] New: kernel 5.2 bpf -- user and kernel-space map reads not synchronized?
Date: Mon, 10 Jun 2019 17:09:00 -0000
X-Bugzilla-Reason: AssignedTo
X-Bugzilla-Type: new
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X-Bugzilla-Product: systemtap
X-Bugzilla-Component: bpf
X-Bugzilla-Version: unspecified
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X-Bugzilla-Who: me at serhei dot io
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D24659

            Bug ID: 24659
           Summary: kernel 5.2 bpf -- user and kernel-space map reads not
                    synchronized?
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: bpf
          Assignee: systemtap at sourceware dot org
          Reporter: me at serhei dot io
  Target Milestone: ---

It appears that the recent kernels (seen on 5.2 rawhide) introduce some tim=
ing
issue between kernel- and user-space map operations.

global value//=3D2
global status//=3D2

probe begin {
  println("BEGIN")
  value =3D 2
  status =3D 2
}

probe kernel.function("vfs_read") {
  status =3D 0
  if (value =3D=3D 2) next
  status =3D 1
}

probe kernel.function("vfs_read") {
  printf("vfs_read\n")
  printf("value %d\n", value)
  printf("status %d\n", status)
  if (status !=3D 2) exit()
}

probe end {
  printf("end\n")
  printf("value %d\n", value)
  printf("status %d\n", status)
  if (status)
    println("END FAIL")
  else
    println("END PASS")
}

In this example, probe end runs too soon after the map update in vfs_read a=
nd
receives an updated value. Changing the second probe vfs_read to probe
timer.s(2) would make the testcase pass.

--=20
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