Re: [PATCH] log: setlogmask(0) can actually result in a system call, don't use it

On Fri, 2 Feb 2024 12:02:19 +1100
David Gibson <david(a)gibson.dropbear.id.au> wrote:

...
  On Fri, Feb 02, 2024 at 12:32:57AM +0100, Stefano
Brivio wrote:
  Before commit 32d07f5e59f2 ("passt, pasta:
Completely avoid dynamic
 memory allocation"), we didn't store the current log mask in a
 variable, and we fetched it using setlogmask(0) wherever needed.
 
 But after that commit, we can just use our log_mask copy instead. And
 we should: with recent glibc versions, setlogmask(0) actually results
 in a system call, which causes a substantial overhead with high
 transfer rates: we use setlogmask(0) even to decide we don't want to
 print debug messages.
 
 Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>   
 
 The basic idea looks fine to me, but there's a curly edge case.  At
 present vlogmsg() works ok before __openlog(), sending the message to
 stderr.  We rely on this because there are die() calls before our
 __openlog(), which go via err() -> vlogmsg().
 
 With this change, before the first __setlogmask(), which occurs right
 after the __openlog() then log_mask is initialized to zero.  Which
 means, AFAICT, that we'll suppress all error messages before that
 point, which isn't great. 
Oops, right, thanks for pointing this out. I had a vague memory of some
side benefit of setlogmask(0) with this regard but I couldn't really
remember and gave up on it -- it was exactly this.

I'm re-posting this with a static initialisation of log_mask now.

-- 
Stefano