POSIX message queues
Quoted 7 lines · Jakub Jelinek
Quoted 3 lines
Speaking of librt - I should not have to link in pthreads and the run-time overhead associated with it (locking stdio etc.) just so I can use
shm_open(). Any chance of fixing this?That overhead is mostly gone in current glibcs (when using NPTL):
a) e.g. locking is done unconditionally even when libpthread is not present (it is just lock cmpxchgl, inlined)
a "lock cmpxchg" is > 100 cycles (according to a recent Linux Journal article from Paul McKenney: 107ns on 700MHz PentiumIII)
But I think you will have benchmarked the alternatives? BTW, what are they?
you suggested naming the syscall number symbols NR_mq_open instead of NR_sys_mq_open. In the stub I want to overload some syscalls (e.g. mq_open) but others not (e.g. mq_timedsend).
How to deal with that?
Quoted 7 lines · Peter Waechtler
Quoted 4 lines · Jakub Jelinek
Quoted 3 lines
Speaking of librt - I should not have to link in pthreads and the run-time overhead associated with it (locking stdio etc.) just so I can use
shm_open(). Any chance of fixing this?
Quoted 7 lines
Quoted 3 lines
That overhead is mostly gone in current glibcs (when using NPTL):
a) e.g. locking is done unconditionally even when libpthread is not present (it is just lock cmpxchgl, inlined)a "lock cmpxchg" is > 100 cycles (according to a recent Linux Journal article from Paul McKenney: 107ns on 700MHz PentiumIII)
Here is exactly what it does on IA-32/i686+:
# define lll_lock(futex) \
(void) ({ int ignore1, ignore2; \
__asm __volatile ("cmpl $0, %%gs:%P6\n\t" \
"je,pt 0f\n\t" \
"lock\n" \
"0:\tcmpxchgl %1, %2\n\t" \
"jnz _L_mutex_lock_%=\n\t" \
".subsection 1\n\t" \
".type _L_mutex_lock_%=,@function\n" \
"_L_mutex_lock_%=:\n\t" \
"leal %2, %%ecx\n\t" \
"call __lll_mutex_lock_wait\n\t" \
"jmp 1f\n\t" \
".size _L_mutex_lock_%=,.-_L_mutex_lock_%=\n" \
".previous\n" \
"1:" \
: "=a" (ignore1), "=c" (ignore2), "=m" (futex) \
: "0" (0), "1" (1), "m" (futex), \
"i" (offsetof (tcbhead_t, multiple_threads)) \
: "memory"); })Quoted 5 lines
you suggested naming the syscall number symbols
NR_mq_openinstead ofNR_sys_mq_open. In the stub I want to overload some syscalls (e.g.mq_open) but others not (e.g.mq_timedsend).How to deal with that?
The syscall is still mq_open, isn't it? So it should be __NR_mq_open.
You simply put the ones where you want to implement them directly in the kernel into syscalls.list (and add sysdeps/generic/mq_*.c stubs; also, mq_{timed,}{receive,send} are cancellation points according to POSIX 2003, so they need to be marked as such). Where you need to do some handling before/after the syscall in userland, you simply write mq_open.c etc. and use INLINE_SYSCALL (or INTERNAL_SYSCALL) in it.
Jakub