[Bug nptl/5192] New: Internal lock in pthread struct is vulnerable to priority inversion

[Bug nptl/5192] New: Internal lock in pthread struct is vulnerable to priority inversion

[matt dot hoosier at gmail dot com]

When most public API operations on pthread_t's execute, the per-thread lock
(struct pthread.lock) is acquired to enforce consistency of the kernel and
userspace data structures.

This can cause a problem when a thread (t1) lowers its own priority and some
other thread (t2, high priority) then immediately becomes runnable as a result
of the priority shift. The scenario would look like this:

* There are three threads: T1 (low prio), T2 (mid prio), T3 (high prio)

* T1 is initially running at some value higher than its permanent priority, to
do some startup work

* T2 is executing some CPU-bound job that is always runnable

* T1 finishes initialization, sets itself to its lower (permanent) priority.
This requires grabbing the locking its own per-thread futex ("lock" in struct
pthread). The syscall to alter scheduling parameters will immediately result in
T2 being put on the CPU, so the lock is not yet dropped.

* T3 eventually needs to do some adjustment of T1's scheduling options. So it
tries to grab T1's per-thread lock, but can't since T1 still holds it because
its scheduling syscall hasn't returned to userspace yet.

* Priority inversion. T2 continues to run unchallenged.

Can the pthread.lock be treated as a PI futex instead of a standard futex, in
order to get priority inheritance and work around this inversion?

I'll attach an example program shortly.

-- 
           Summary: Internal lock in pthread struct is vulnerable to
                    priority inversion
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: drepper at redhat dot com
        ReportedBy: matt dot hoosier at gmail dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: arm-none-linux-gnueabi


http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[matt dot hoosier at gmail dot com]

------- Additional Comments From matt dot hoosier at gmail dot com  2007-10-17 15:52 -------
Created an attachment (id=2048)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=2048&action=view)
Example to illustrate priority inversion in NPTL pthread internals


-- 


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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[ismail at pardus dot org dot tr]

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           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ismail at pardus dot org dot
                   |                            |tr


http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[spcurry5 at linux dot vnet dot ibm dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|drepper at redhat dot com   |spcurry5 at linux dot vnet
                   |                            |dot ibm dot com
             Status|NEW                         |ASSIGNED


http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[spcurry5 at linux dot vnet dot ibm dot com]

------- Additional Comments From spcurry5 at linux dot vnet dot ibm dot com  2008-04-25 00:27 -------
Example test case (priority-inversion.c) confirmed. A priority inversion occurs,
causing the high-priority third thread to wait on the low-priority first thread.

-- 


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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[spcurry5 at linux dot vnet dot ibm dot com]

------- Additional Comments From spcurry5 at linux dot vnet dot ibm dot com  2008-04-25 01:26 -------
I have a few points worth noting:

(1) I was unable to find anywhere in the POSIX specification that states the
per-thread mutex must not cause a priority inversion.
(2) Changing the pthread implementation so that every thread has a
priority-inheritance mutex, instead of a standard mutex, would cause some
performance loss due to the extra overhead associated with using
priority-inheritance mutexes. 
(3) The priority inversion situation you have described does not cause any of
the threads to hold onto a resource indefinitely, thereby preventing some other
thread from ever making forward progress. All threads eventually make forward
progress, therefore, this is more of a performance issue than a correctness
issue. So I am marking this bug as an enhancement.

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           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[rsa at us dot ibm dot com]

------- Additional Comments From rsa at us dot ibm dot com  2008-04-25 20:14 -------
BZ flagged for Ulrich's attention:

http://www.sourceware.org/ml/libc-alpha/2008-04/msg00094.html

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http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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[Bug nptl/5192] Internal lock in pthread struct is vulnerable to priority inversion

[spcurry5 at linux dot vnet dot ibm dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|spcurry5 at linux dot vnet  |drepper at redhat dot com
                   |dot ibm dot com             |
             Status|ASSIGNED                    |NEW


http://sourceware.org/bugzilla/show_bug.cgi?id=5192

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