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[Bug nptl/5192] New: Internal lock in pthread struct is vulnerable to priority inversion
- From: "matt dot hoosier at gmail dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: 17 Oct 2007 15:10:51 -0000
- Subject: [Bug nptl/5192] New: Internal lock in pthread struct is vulnerable to priority inversion
- Reply-to: sourceware-bugzilla at sourceware dot org
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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