Hello,
I am trying to understand how NPTL works wrt cancellation.
Please, could someone provide brief explanation here ?
So far I hit into this:
a)
nptl/forward.c
FORWARD_NORETURN(__pthread_unwind,
...
INTERNAL_SYSCALL_DECL (err);
INTERNAL_SYSCALL (kill, err, 1, SIGKILL);
But usually "int kill(pid_t pid, int sig)" holds.
It looks like pid 9 is killed by random signal ...
b)
nptl/cancellation.c: __pthread_disable_asynccancel (int oldtype)
/* We cannot return when we are being canceled. Upon return the
thread might be things which would have to be undone. The
following loop should loop until the cancellation signal is
delivered. */
while (__builtin_expect ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
== CANCELING_BITMASK, 0))
{
lll_futex_wait (&self->cancelhandling, newval, LLL_PRIVATE);
newval = THREAD_GETMEM (self, cancelhandling);
}
Can this ever return, or will it only again and again call kernel,
until thread is somehow terminated ?
c)
nptl x86_64 specific cancellation.S
The comment says:
/* It is crucial that the functions in this file don't modify registers
other than %rax and %r11. The syscall wrapper code depends on this
because it doesn't explicitly save the other registers which hold
relevant values. */
But later on:
3: andl $(TCB_CANCELING_BITMASK|TCB_CANCELED_BITMASK), %eax
cmpl $TCB_CANCELING_BITMASK, %eax
je 4f
1: ret
/* Performance doesn't matter in this loop. We will
delay until the thread is canceled. And we will unlikely
enter the loop twice. */
4: mov %fs:0, %RDI_LP
movl $__NR_futex, %eax
xorq %r10, %r10
addq $CANCELHANDLING, %rdi
LOAD_PRIVATE_FUTEX_WAIT (%esi)
syscall
movl %fs:CANCELHANDLING, %eax
jmp 3b
Again, can this ever return ?
Or the modification of registers is not problem any more ?
And moreover, the third agument of futex (rdx, old value) is not set before
syscall at all ...
Am I mising some obvious ?
Petr