Thread flags modified without set_thread_flag() (non atomically)

Mathieu Desnoyers
2007-02-26T20:10:54Z

Hi,

Looking into the thread flags, I found out that some architecture
specific kernel functions (in 2.6.20) sets the thread flags with non
atomic operation.

A good way to list the most trivial : grep -r TIF_ * | grep =

Some examples follows. If, for instance,
x86_64/kernel/process.c:flush_thread is called from an exec system call,
it will do the following :

x86_64/kernel/process.c: t->flags ^= (_TIF_ABI_PENDING |
_TIF_IA32);
x86_64/kernel/process.c: t->flags &= ~_TIF_DEBUG;

void flush_thread(void)
{
struct task_struct *tsk = current;
struct thread_info *t = current_thread_info();

if (t->flags & _TIF_ABI_PENDING) {
t->flags ^= (_TIF_ABI_PENDING | _TIF_IA32);
if (t->flags & _TIF_IA32)
current_thread_info()->status |= TS_COMPAT;
}
t->flags &= ~_TIF_DEBUG;
....

As long as the flags are only updated by the thread itself at this
moment, it seems safe, but if other updates coming from other threads
are expected, wouldn't it result in a bad behavior ?

i.e if resched_task ia being called by another CPU at the same time for
this specific thread would set the TIF_NEED_RESCHED flag, but it could
be overwritten by the non-atomic modification in flush_thread.

And about this specific flush_thread, I am puzzled about the t->flags ^=
(_TIF_ABI_PENDING | _TIF_IA32); line. The XOR will clearly flip the
_TIF_ABI_PENDING bit to 0, and very likely set _TIF_IA32 to the opposite
of its current value. Why does this change need to be written atomically
(can other threads play with these flags ?) ?

Mathieu

Other examples :

sparc64/kernel/ptrace.c: if
((task_thread_info(child)->flags & _TIF_32BIT) != 0) {
sparc64/kernel/process.c: t->flags ^= (_TIF_ABI_PENDING |
_TIF_32BIT);
sparc64/kernel/process.c: t->flags &= ~_TIF_PERFCTR;

sparc/kernel/process.c: current_thread_info()->flags &=
~_TIF_USEDFPU;
sparc/kernel/process.c: current_thread_info()->flags &=
~_TIF_USEDFPU;
sparc/kernel/process.c: current_thread_info()->flags &=
~_TIF_USEDFPU;
sparc/kernel/process.c: current_thread_info()->flags &=
~(_TIF_USEDFPU);
sparc/kernel/traps.c: current_thread_info()->flags |= _TIF_USEDFPU;
sparc/kernel/traps.c: task_thread_info(fpt)->flags &= ~_TIF_USEDFPU;

powerpc/kernel/process.c: t->flags ^= (_TIF_ABI_PENDING |
_TIF_32BIT);

ia64/kernel/mca.c: ti->flags = _TIF_MCA_INIT;

avr32/kernel/ptrace.c: ti->flags |= _TIF_BREAKPOINT;
avr32/kernel/ptrace.c: ti->flags |= TIF_SINGLE_STEP;