Re: [Qemu-devel] [PATCH] fix SMB-related lockup on some systems

[Thiemo Seufer <ths@networkno.de>]

Jörg Walter wrote:
> Hi!
> 
> I have found a bug where qemu would sit in an endless loop whenever "-smb" was 
> enabled and accessed. It is probably the same problem some people in the user 
> forums talk about, and here's my analysis and fix:
> 
> On glibc systems with NPTL, fork() is not atomic with regard to signals, while 
> on non-NPTL-systems, it is. This behaviour is considered to be correct by the 
> libc developers, as no relevant spec forbids this behaviour.
> 
> See this thread for details:
> http://sourceware.org/ml/libc-hacker/2007-02/msg00009.html
> 
> In qemu, accessing the SMB ip-address causes the slirp code to issue a fork in 
> slirp/misc.c, which hangs, as we are in mid-emulation and SIGALARM signals 
> come in at a high rate, probably triggering the above mentioned behaviour.
> 
> This patch solves the problem by temporarily blocking all signals until the 
> fork is over. It doesn't unblock signals in the child, as I assume that 
> executing the server program will care for that anyways. It works for me, 
> finally I can access "-smb"-folders again.
> 
> -- 
> CU
>   Jörg

> --- slirp/misc.c	2007-08-27 10:30:20.000000000 +0200
> +++ slirp/misc.c.new	2007-08-27 10:29:50.000000000 +0200
> @@ -313,6 +313,7 @@
>  	int opt;
>          int master;
>  	char *argv[256];
> +	int mask;
>  #if 0
>  	char buff[256];
>  #endif
> @@ -346,8 +347,10 @@
>  		}
>  	}
>  	
> +	mask = sigsetmask(~0);
>  	switch(fork()) {
>  	 case -1:
> +	 	sigsetmask(mask);
>  		lprint("Error: fork failed: %s\n", strerror(errno));
>  		close(s);
>  		if (do_pty == 2)
> @@ -426,6 +429,7 @@
>  		exit(1);
>  		
>  	 default:
> +	 	sigsetmask(mask);
>  		if (do_pty == 2) {
>  			close(s);
>  			so->s = master;

Could you rewrite this using pthread_sigmask, to make it thread safe?


Thiemo