Re: [PATCH v2 0/2] vfs: Define new syscall getumask.

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Apr 13, 2016, at 8:57 AM, Richard W.M. Jones rjones@redhat.com wrote:

> v1 -> v2:
> 
> - Use current_umask() instead of current->fs->umask.
> 
> - Retested it.
> 
> ----------------------------------------------------------------------
> 
> It's not possible to read the process umask without also modifying it,
> which is what umask(2) does.  A library cannot read umask safely,
> especially if the main program might be multithreaded.
> 
> This patch series adds a trivial system call "getumask" which returns
> the umask of the current process.

In addition to this system call, we could extend a variation of my
thread_local_abi system call (https://lkml.org/lkml/2016/4/4/455)
(could be without features flags, or an entirely new system call
specifically for a umask cache) to register a "current umask" cache
located in a TLS area.

Basically, reading the current umask value would be a simple load from
a TLS variable. This could also allow quickly blocking and unblocking
signal delivery from user-space by storing a mask to this TLS area.

The kernel could then look into the signal mask in this TLS area whenever
it needs to deliver a signal (assuming this code path can take
user-space faults), in addition to the mask kept within the
task struct.

This "tls cache" idea could also apply to setting a CPU affinity to the
currently running CPU for short user-space critical sections.

The benefit here is to get _very_ fast operations on the thread umask
and cpu affinity.

Are those ideas too far-fetched ?

Thanks,

Mathieu

> 
> Another approach to this has been attempted before, adding something
> to /proc, although it didn't go anywhere.  See:
> 
>  http://comments.gmane.org/gmane.linux.kernel/1292109
> 
> Another way to solve this would be to add a thread-safe getumask to
> glibc.  Since glibc could own the mutex, this would permit libraries
> linked to this glibc to read umask safely.
> 
> I should also note that man-pages documents getumask(3), but no
> version of glibc has ever implemented it.
> 
> Typical test script:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <linux/unistd.h>
> #include <sys/syscall.h>
> 
> int main(int argc, char *argv[])
> {
>  int r = syscall(329);
>  if (r == -1) {
>    perror("getumask");
>    exit(1);
>  }
>  printf("umask = %o\n", r);
>  exit(0);
> }
> 
> $ ./getumask
> umask = 22
> 
> Rich.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com