Re: [PATCH] PPC32: cancel syscall restart on signal delivery

[Linus Torvalds]

On Fri, 14 Nov 2003, Linus Torvalds wrote:
> 
> On x86, for the old calling conventions (ie "int 0x80"), that meant that 
> the interrupt window was literally a single instruction. For the new one 
> ("sysenter") it was two instructions.

Actually, three instructions for "sysenter": the "movl %esp,%ebp" and the
actual "sysenter" instruction itself are the main restart sequence, but to 
make it look the same as the legacy code from a kernel restart standpoint 
there is also a short backwards jump there.

Anyway, you can't even trigger it with the single-step flag, since the
kernel will disable single-stepping in sysenter (and it only gets
re-enabled once the system call is done - including retries).

Since it literally requires an external interrupt to happen, it would be 
pretty hard to create a test-case for. Maybe a dual-CPU setup with the 
other CPU sending a signal (and the interrupt is the resultant IPI) at 
just the right moment..

		Linus

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