Understanding interrupted system calls

[Joe Marcus Clarke]

On Sun, 2007-09-02 at 14:17 +1000, Bruce Evans wrote:
> >From Jilles' previous reply:
> 
> >>> The problem seems to be the following code in
> >>> src/sys/dev/syscons/syscons.c, in case VT_WAITACTIVE in scioctl():
> >>> 
> >>> 	while ((error=tsleep(&scp->smode, PZERO|PCATCH,
> >>> 			     "waitvt", 0)) == ERESTART) ;
> >>> 
> >>> If a signal is caught and system call restart is enabled for that
> >>> signal, this makes it spin in a tight loop, waiting in vain for the
> >>> signal to go away.  The idea of ERESTART is that the syscall function
> >>> returns it and then the signal handler is entered.  If and when the
> >>> signal handler returns, it will return to the system call instruction,
> >>> restarting it (perhaps this is optimized to avoid the switch to userland
> >>> and back).  With EINTR, the signal handler would return to directly
> >>> after the system call instruction.
> >>> 
> >>> The fixed version would then be
> >>> 
> >>> 	error = tsleep(&scp->smode, PZERO|PCATCH, "waitvt", 0);
> 
> I think this is right.  The kernel should never loop on ERESTART like this.
> Please fix the remaining style bug in it (missing spaces around binary
> operator).

Bruce, I didn't know if you saw my fix, if it needs more work, or if
you're going to commit it?  I'd really like to get this fixed before
7.0.  Should I open a PR to track it?  Thanks.

http://www.marcuscom.com/downloads/syscons.c.diff
http://www.marcuscom.com/downloads/pcvt_ext.c (-STABLE only)

Joe

-- 
Joe Marcus Clarke
FreeBSD GNOME Team      ::      gnome at FreeBSD.org
FreeNode / #freebsd-gnome
http://www.FreeBSD.org/gnome
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