Messages in this thread | | | Date | Mon, 11 May 2009 22:47:43 +0200 | From | Vitaly Mayatskikh <> | Subject | Re: [PATCH 1/5] Split wait_noreap_copyout() |
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At Mon, 11 May 2009 14:17:08 +0200, Ingo Molnar wrote:
> > if (put_user(signal, &infop->si_signo) || > > put_user(0, &infop->si_errno) || > > put_user((short)why, &infop->si_code) || > > put_user(pid, &infop->si_pid) || > > put_user(uid, &infop->si_uid) || > > put_user(status, &infop->si_status)) > > return -EFAULT; > > For best assembly code this should generally be written as a series > of: > > __uaccess_err |= __put_user(x, ptr); > __uaccess_err |= __put_user(y, ptr); > __uaccess_err |= __put_user(z, ptr); > > As this generates non-dependent, compressed, branch-less code.
Yeah, my first intention was to eliminate a lot of branches in one place. It's terrible for CPU pipeline, I bet.
> See the (new) put_user_try / put_user_ex() / put_user_catch() > abstraction in arch/x86/include/asm/uaccess.h, and how all the x86 > signal code makes use of that to optimize such patterns of per field > user copies.
So, there's catch block to handle GPF and the code inside of `try' block is still branch-less, right? I was thinking of minimized version of struct siginfo (up to si_uid) and copying it with single copy_to_user(), but the idea with try/catch is definitely much better.
-- wbr, Vitaly
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